A user interface prototype is a hypothesis — a candidate design solution that you consider for a specific design problem. The most straightforward way to test this hypothesis is to watch users work with it.

There are many types of prototypes, ranging anywhere between any of these pairs of extremes:

Single page vs. multipage with enough menus, screens, and click targets that the user can completely finish a task
Realistic and detailed vs. hand-sketched on a piece of paper
Interactive (clickable) vs. static (requiring a person to manipulate different pages and act as a “computer”)
The choice of prototype will vary greatly depending on goals of the testing, completeness of the design, tools used to create the prototype, and resources available to help before and during the usability tests. But, whatever prototype you may use, testing it will help you learn about users’ interactions and reactions, so you can improve the design.