A prototype makes an idea tangible enough to react to. It can be a paper sketch, a clickable mockup, or a rough working model. Fidelity is chosen to fit the question: a low-fidelity prototype tests a concept, a high-fidelity one tests interaction and detail.
The point is learning, not polish. Each prototype exists to answer a specific question and to expose flaws while they are still cheap to fix. Prototyping pairs naturally with testing, turning opinion into observed behaviour and shortening the distance between an idea and evidence.