A user can have multiple sessions. Sessions can occur on the same day or over several days, weeks or months. When a session ends, there is an opportunity to start another session. By default, Google Analytics ends a session when there hasn’t been any activity from the user in the past 30 minutes. This is based on the session timeout settings. The next time Google Analytics detects a hit from the user, a new session is created. For example, a user lands on a page and idles for 2 hours. After 30 minutes, the session will end. When the user comes back to their computer and interacts with the page again, a new session will be created. Two sessions were counted by Google Analytics. You can change the session timeout based on your business needs from 1 minute to 4 hours.
To change your session timeout, go to Admin and browse to the property you want to change the session timeout for. Click on Tracking Info and click on Session Settings. Here you’ll be able to change the session timeout and campaign timeout.