Google Analytics is a free tool that allows you learn information about who is visiting your website. You can learn a number of things like how many people looked at your website, which web pages they viewed, for how long, which page they started on and which page they left.
You can also learn how they found your website and if they completed any of your actions like submitting a form. There is a lot of data that you can analyze.
The steps to add Google Analytics to your website involves added a small piece of code to each web page.