A prototype is an early, testable version of a product idea. It might be a paper sketch, a set of linked screen mockups, or a basic working model. It helps a team see whether an idea makes sense before spending time building the finished product.
Think of trying a paper layout before constructing a whole room. A prototype can be rough or detailed depending on what the team wants to learn. It is not necessarily the final product, and it may be changed or discarded after people try it.