A message consumer is a program that receives messages from a queue and handles them. The queue holds the messages; the consumer does the work they describe.
For example, when someone places an order, one consumer might prepare a confirmation email. It takes the task from the waiting line and processes it.
If the consumer is busy or stopped, messages may wait in the queue. The app should have a plan for what happens if handling a message fails, so important tasks are not silently lost.