Ordinals
Satoshis (SATS) in Bitcoin are numbered using an ordinal system, which makes it possible to track and move specific units. Ordinals inscribe a serial number on a satoshi, bitcoin’s lowest unit of value, along with the ordinal information. This number is added to a specific portion of the bitcoin transaction.
With this knowledge, you may validate that the tokens you are using are legitimately yours and prevent its double use. But, above all, ordinals are useful for protecting NFT market trading since they store data (text, HTML, pictures, etc.) directly on-chain as opposed to linking to a file hosted elsewhere.
Ordinals are commonly associated to BRC-20 tokens, because BRC-20 tokens use their protocol. But there are millions of ordinals out there, and only about 14,000 BRC-20 tokens.