What it means
The pool's invariant check failed: after the swap, the pool would hold less value than before, which the contract forbids. In practice this almost always involves a token that takes a fee on transfer or otherwise misreports amounts, breaking the pool's arithmetic.
How to fix it
Use the dapp's fee-on-transfer supporting swap function if it has one (many routers have a separate path for such tokens), or trade the token on a venue built for it. Plain V2-style swaps of fee-on-transfer tokens fail here by design.