A message queue (MQ) is an asynchronous and reliable way of moving data around your system. It is useful for offloading work from your web application to a background service, but can also be used to update multiple parts of your system concurrently. For example, distributing cache invalidation data to all of your web servers.
MQs add complexity and we will cover managing this in Chapter 10, The Downsides of Performance-Enhancing Tools. However, they can also assist in implementing a microservices architecture where you break up your monolith into smaller parts, interfaced against contracts. This can make things easier to reason about within large organizations, where different teams manage the various parts of the application. We will discuss this in more detail in the next chapter, as queues aren't the only way of implementing this style of architecture. You can build microservices with many transport technologies, for example HTTP APIs can be used.