An anti-corruption layer is a translation boundary that protects your domain model from leaking concepts from a legacy/external system. You use it when integrating with something that has a different model or ugly API, so you keep your core clean and map data in one place.
Expanding on the short answer — what usually matters in practice:
A tiny example (an explanation template):
// Example: discuss trade-offs for "anti-corruption-layer-(acl):-what-is-it-and-when"
function explain() {
// Start from the core idea:
// An anti-corruption layer is a translation boundary that protects your domain model from le
}