A flash loan attack is an abuse of the smart contract security of a particular platform in which an attacker usually borrows a lot of funds that don’t require collateral. They then manipulate the price of a crypto asset / NFT on one exchange and quickly resell it on another one.
The process is swift, and the attacker repeats the process multiple times before finishing and leaving without a trace.
https://medium.com/coinmonks/nft-flashloan-ape-coin-d6065c7dd956