The Entry Point into a Service
A handler is the entry point of a message into the business logic of a service. It receives instructions from other services, apps, and clients. You might think of them as controllers in MVC terms, but that's a very loose comparison.
This handler does deposits to a bank account.
A handler receives a command, does its work, and when it's done with that work, it reports the status and outcome of that work by publishing an event.
A handler might also respond (or react) to events by other services, or it might respond to events published by its own service (when a service calls itself).
For more, see the handlers user guide.