Backpressure is a way for a slower part of a system to signal that it cannot handle more work right now. The faster part can pause or reduce how quickly it sends new messages.
Imagine pouring water into a bottle. If the bottle fills faster than it can drain, you stop pouring before it overflows. Backpressure plays a similar role between computer programs.
This helps prevent a growing pile of work from overwhelming a service. The system still needs a plan for messages that must wait, such as storing them in a queue.