As you can see in the previous chart, a prototype is the next logical step from a mockup. The key distinction between a prototype and a mockup is that a prototype is interactive. However, it's still not the finished product. It only has very limited functionality, and the interactions are only simulated.
In other words, prototypes don't actually execute the process they are designed for, but they do give the user an instant response, which demonstrates how the real, fully developed system would behave. So, think of a prototype as a mini demo.
Similar to the previous design techniques, the key goal is to obtain user feedback, which helps you to make more informed design decisions and therefore reduce the risk of building something that's not what the customer wanted.
Whereas wireframes and mockups are only for illustrative purposes...