Google Analytics lets you gather information on the visitors who visit your website. You don’t get personal details like names and addresses but you do get important and valuable information about what pages people visit and land on, how long they spend on each page, how many pages they visit etc.
This is great information and means you can see what is performing well and what is not. You can modify certain areas of your website to make them perform better, inter-link pages so you can pass the traffic from one good performing page to a not as good a performing page and other things like this.
One of the main uses for Google Analytics is just to see how your website traffic is growing. Ultimately the success of your website is down to the number of visitors it attracts, so monitoring this is a great way to see the fruits of your labour.
The video below shows how to add Google Analytics to your website.
Quick link to Google Analytics: https://www.google.com/analytics/