A prototype is a draft version of a product that allows you to test it.
A prototype can help you to understand different things such as the flow of the user journey, the overall design concept and how a specific feature might work. A prototype allows you to explore ideas and test the product quickly before jumping into development. Prototypes can be low fidelity (such as sketches and post-its), mid fidelity (such as wireframes) or high fidelity (fully designed pages).