A file identified by what it is, not where it lives
IPFS, the InterPlanetary File System, is a protocol for storing and sharing files across a distributed network of computers. Its core idea is content addressing: instead of identifying a file by a location, like a URL that points to a specific server, IPFS identifies a file by a cryptographic hash generated from the file's actual content.
That hash, often called a content identifier or CID, is deterministic. Run the same file through the hashing process anywhere in the world and you get the exact same identifier every time. This means the same file always resolves to the same CID no matter which computer is storing it, and if even a single byte of the file changes, the resulting hash changes completely, so a CID also acts as a built-in integrity check.