MVPs
A working first version. Small enough to build quickly, real enough to put in front of actual users.
An MVP isn't a demo. People need to be able to sign up and use it properly, or whatever you learn from it isn't worth much.
Most of the work is deciding what to leave out. We start from the question you're trying to answer and cut anything that doesn't help answer it.
What you end up with is something you can keep building on, not a throwaway you'll rewrite the week it gets traction.