Definition: A test stub is a stand-in for a real component or interface that returns pre-set responses, so code can be developed or tested without the real thing. Stubs often rely on assumed data.
So what? Stubs make it possible to move quickly when a dependency isn't ready. The risk is building on assumptions that turn out to be wrong, which real examples would have exposed sooner.
Examples: Building a stub payment gateway that returns assumed messages while the real gateway is unavailable. Replacing a slow external service with a stub during automated tests.
