Backpressure
A mechanism by which a component under load tells its callers to slow down, rather than accepting work it cannot complete.
Without backpressure, a system under overload buffers. Queues grow, memory fills, latency climbs until every request in flight has already timed out at the client, and the system spends all its capacity producing responses nobody is waiting for. This is congestion collapse, and it is worse than refusing work.
Backpressure is the alternative: bounded queues, explicit rejection (HTTP 429, 503 with Retry-After), TCP-style flow control, or pull-based consumption where the consumer asks for work at the rate it can handle.
Rejecting work early is a feature. The instinct to accept everything and "try our best" is the instinct that turns a busy hour into an outage.