A BFF is a backend tailored to one frontend (web, mobile, etc.). It helps when different clients need different data shapes, when you want to reduce chatty calls from the UI, or when you need a safe place to aggregate multiple service calls into one API for that client.
Expanding on the short answer — what usually matters in practice:
A tiny example (an explanation template):
// Example: discuss trade-offs for "bff-(backend-for-frontend):-what-is-it-and-when-"
function explain() {
// Start from the core idea:
// A BFF is a backend tailored to one frontend (web, mobile, etc.). It helps when different c
}