a snapshot is basically a "picture" of the current status of the wallets,
overall, they delete old history of all the movements a wallet did, and just remember the current status of the wallet,

its like a bank account, the bank remove all your movements but remembers your actual balance, thats all it cares about,

this way the integrity of the network is preserved and the database is Waay smaller than blockchain based ones,

still some nodes, preserve the full history just in case anyone need access to this (usually) useless data.