Ordinals is the protocol is a convention for numbering and transferring individual satoshis on the Bitcoin network. It was promoted by former Bitcoin Core contributor Casey Rodarmor.
This effectively enables Bitcoin-native on-chain NFTs. The capability was always there yet was made much easier by the Taproot upgrade that took place in 2021.
According to a press release, Ords “is a wallet and explorer that allows tracking the location of specific satoshis and their ordinal numbers - assigned by the Ordinals protocol - as well viewing, creating, and transferring inscriptions, that is, individual satoshis inscribed with arbitrary content."
The introduction of the Ordinal protocol brings NFTs to the Bitcoin network. It allows for the including of images, videos, and HTML in Bitcoin transactions and the assigning of an individual satoshi. This is all done on-chain without the next for sidechains or a separate token.
Update
On February 18, 2023, an Australian software engineer by the name of Anthony Guerrera posted a repository to GitHub that forked the Bitcoin Ordinals protocol to Litecoin. This brings the same ability of including NFTs in the Litecoin transactions.
Litecoin was originally a fork of Bitcoin.