A session is a concept in Google Analytics that is defined as a period of time during which a user is actively engaged with your website or application. This means that interactions are all tied to sessions. Google sometimes describes a session as a “container” for the actions that a user takes on your website. For example, one session might include multiple pageviews, two button clicks, a comment, and a transaction (purchase).
You should keep in mind that a single user can open multiple sessions. It may help for you to understand how sessions are closed: (1) a session will end after 30 minutes of inactivity, or arbritarily at midnight, or (2) if a user arrives via one campaign, leaves the website, then comes back via a different campaign.
Every time a user takes another action on a website, the 30-minute clock is reset, and they are given another 30-minutes before the session ends.
The way sessions are calculated can cause some problems when you are doing analysis of your website. For instance, if someone visits your website, begins to make a purchase, and then stops for 35 minutes before deciding to finish making the purchase, that purchase will be broken into two sessions. Furthermore, it will look like the second session had a “landing page” that was halfway through the purchase funnel. It is important to have a thorough understanding of sessions in order to get the most out of Google Analytics reports.