Open source is something that people can openly share or alter because the design was for public use. It originally started with software that was developed under a license where the copyright holder grants users the rights to use it as they see fit.
This expanded over the decades to become the "open source way", a stance taken by many developers that create programs for public use.
It is commonly thought of as open source software (OSS).
History
This is a concept that dates back to 1983. It sprung from an ideological movement informally founded by Richard Stallman, a programmer at MIT.
He felt software should be accessible to programmers so they could modify it as they wished. From this they could seek to understand and, hopefully, improve upon it.
Stallman began releasing free code under his own license.
The new approach to software creation took hold and eventually led to the formation of the Open Source Initiative in 1998.
GNU Public License
The GNU General Public License is a free, copyleft license for software and other kinds of works.
The licenses for most software and other practical works are designed to take away your freedom to share and change the works. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change all versions of a program--to make sure it remains free software for all its users. We, the Free Software Foundation, use the GNU General Public License for most of our software; it applies also to any other work released this way by its authors. You can apply it to your programs, too.
When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for them if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs, and that you know you can do these things.
To protect your rights, we need to prevent others from denying you these rights or asking you to surrender the rights. Therefore, you have certain responsibilities if you distribute copies of the software, or if you modify it: responsibilities to respect the freedom of others.
For example, if you distribute copies of such a program, whether gratis or for a fee, you must pass on to the recipients the same freedoms that you received. You must make sure that they, too, receive or can get the source code. And you must show them these terms so they know their rights.
Developers that use the GNU GPL protect your rights with two steps: (1) assert copyright on the software, and (2) offer you this License giving you legal permission to copy, distribute and/or modify it.
For the developers' and authors' protection, the GPL clearly explains that there is no warranty for this free software. For both users' and authors' sake, the GPL requires that modified versions be marked as changed, so that their problems will not be attributed erroneously to authors of previous versions.
Some devices are designed to deny users access to install or run modified versions of the software inside them, although the manufacturer can do so. This is fundamentally incompatible with the aim of protecting users' freedom to change the software. The systematic pattern of such abuse occurs in the area of products for individuals to use, which is precisely where it is most unacceptable. Therefore, we have designed this version of the GPL to prohibit the practice for those products. If such problems arise substantially in other domains, we stand ready to extend this provision to those domains in future versions of the GPL, as needed to protect the freedom of users.
Finally, every program is threatened constantly by software patents. States should not allow patents to restrict development and use of software on general-purpose computers, but in those that do, we wish to avoid the special danger that patents applied to a free program could make it effectively proprietary. To prevent this, the GPL assures that patents cannot be used to render the program non-free.
This license distinction means the source code, and modifications, are outside the control of corporations.
Internet
The Internet changes the scope of open source.
Not only did it incorporate it into many of the models, it also provided open collaboration between people all over the world.
Early on, in the 1950s, software coders shared what they created with others to learn from each other and expand the field of computing. A lot of this was done in academia although commercial developers did the same.
Over time, corporations realized the value of intellectual property and siloed systems developed. This is true for many aspects of the Internet today.
Websites such as Github emerged to give developers a repository where code could be shared and stored. This includes both closed and open source but did enhance collaboration.
Blockchain
The introduction of Bitcoin by the anonymous Satoshi Nakamoto changed many things.
Blockchains, for the most part, are open source meaning that anyone can take the source code and modify it. This is evidenced by the forks of Bitcoin that took place over the years.
Ethereum and the EVMs followed the same pattern.
When a blockchain is forked, there are two databases created. One option is to have the new chain go back to the genesis block of the original one. Another is to simply start the database from the time of the fork.
Things such as block size, block production time, and consensus mechanism. It is up to the new developers to decide what they want to change.
Any updates to the software are not part of the first chain unless those developers adopt those changes. Post fork, the databases and the source code running it are completely separate.
With blockchain, a new cryptocurrency is required. The first coin is tied to the original chain.
In this instance, project teams will often do an airdrop, where the original holders of the coin will get the new one. This is based upon the wallets and will duplicate it although the ability to alter that is within the rights of the new developers.