A buffer between parts of a system: one service posts a job, another picks it up when ready, so slow or failing work does not block the request that triggered it.
It is what keeps a product responsive under load and what stops one failing integration from taking the checkout down with it.
We move anything slow or external — emails, exports, third-party calls, webhooks — behind a queue with retries, backoff, and a dead-letter queue that a human actually monitors.