A goal is something you want your visitors to do, where you'd like to track and measure how many of your visitors do that thing.
It might be buying a product, or signing up for a newsletter, or downloading a resource or even just visiting a specific page on your site. Whatever it is you want your visitors to do, you can set as a goal.
You can have more than one goal, too. If you want to easily see what percentage of your visitors sign up for your newsletter and you also want to track what percentage download a white paper, you can set them both up as goals.
Google has an excellent help section
describing all about how goals work and how to set them up. I suggest you review this, and if you still need clarification after that, come back and ask your specific question(s) and we'll do our best to help.