Message queue
A message queue is a buffer that receives asynchronous tasks — sending emails, generating PDFs, processing images — and distributes them to independent workers, decoupling request reception from its processing and preventing bottlenecks under load.
In practice
Without a queue, a web server that sends 500 emails synchronously during a newsletter dispatch blocks all other requests for several minutes. With RabbitMQ, Redis Queue (BullMQ), or SQS, the dispatch is queued and handled by dedicated workers without impacting site availability. BullMQ is the simplest option for a Node.js project: five lines of code to publish a task, five to consume it.