The stub: component provides a simple way to stub out any physical endpoints for easy testing. Just add stub: in front of any endpoint URI in order to stub out the endpoint. This is useful in development where you might wish to try a route without needing to connect to a specific SMTP or HTTP endpoint.
Internally the Stub component creates VM endpoints. The main difference between Stub and VM is that VM will validate the URI and parameters you give it, so putting vm: in front of a typical URI with query arguments will usually fail. Stub won't though as it basically ignores all query parameters to let you quickly stub out one or more endpoints in your route temporarily.