One of the key part of a user centered development process is to test your ideas with them. To do so you will have to create prototypes.
Nowadays, it's common to see processes with short iterative cycles where you prototype fast, test and improve often, especially in web and interaction design. It's often better (and faster) to just make a prototype and test it to ensure you're going somewhere. Prototypes are also a good way to help people discussing and understand your idea.
A prototype is not a full version, it can be really low fidelity (lo-fi) or high fidelity (hi-fi), it can focus only on 1 action, on aesthetics or on the whole experience.
There are 3 kinds of prototypes: Feel and Look prototypes, Role prototypes (focus on the experience, the functionalities) and finally Implementation prototypes (the technical part, how it works).
In product design: cardboard, clay, foam, wood or even 3D printing will often be enough to test a lot of aspects and all of them are really easy to use, so no excuses!
On the photo here you can see a lo-fi prototype made to test the ergonomics of a hand held gardening tool: a bottle, some plastic, a piece of wood and foam were enough to test with users, but also to help them coming up with more ideas.
Making the prototype by itself can also help you, the designer improve your idea. Sometimes, when stuck on a problem, the best you can do is to go to the workshop/take some clay and try to build something. The same applies to a co-creative session: it can be easier for participants to think about concepts, explain them if they can prototype fast and easily.

In Interaction design you will create paper prototypes, digital mockups and in Services especially you can use Storyboards to visualize the whole experience.
Low fidelity prototype to test different positions of handle on a gardening product.