files are divided in blocks - typically 128 KiB each, but possibly larger for larger files. Each device calculates the hash (cryptographic checksum) of all blocks making up a file and informs its peers about the file contents. When Syncthing gets an index update from a peer device, containing a new block list, it compares the new block list with the one it already has in the index. If there are differences that means the file contents have changed and we should synchronize the file.