A session is when a visitor makes a series of interactions on your site. It starts with the first page they visit and ends when there's a period of inactivity that lasts longer than 30 minutes.
For example, let's say a visitor finds your website through Google and lands on your homepage. As a result, this session begins with the homepage. Let's say they visit your product page and leave for a coffee break. Because the coffee break lasted for more than 30 minutes, and the visitor comes back to the site to visit the pricing page and product page, he has triggered a second session.
The session will end when the visitor has been inactive for over 30 minutes. If the visitor returns to your site after 30 minutes, their activity will be recorded as a new, separate session.
Let's look at another example. Let's say a visitor lands on your homepage. The session begins at 2PM. Then, the visitor goes to the product page, leaves for a snack break and leaves the product page open. If the visitor returns to the site and was inactive for less than 30 minutes, their activity will be recorded within the same session.