A subnet is quite simply a division of the network. The network can be made up of 2 or 3 subnets. The technology that supports this is vlans.
Why do we use subnets?
Subnets are used to minimize a broadcast domain but most importantly this is used in a large environment where there are 1000's of users and servers and splitting these up is for the greater good of reducing congestion and increasing speed.
Subnetting is a bit more complex to use and it is advised that you complete a tutorial to understand this and implement.
There are many articles online that provide detailed explainations however this little overview makes you understand it's need for you to have this in you environment.
Thanks for reading.
Regards,
@trev03