Page Sessions: Page session in analytic is basically the user interactions on your webiste/blog for a limited time period. When we open a website, we create one session and as soon as we start exploring the website and open multiple page, events, do transactions & fill forms we create multiple sessions. In simple words, a single user can create multiple sessions on website which we can see in google analytics. Basically there are two major methods by which a session ends:
Time-based expiration & Campaign Change. In time based expiration, a session could expire after 30 minutes of inactivity or at midnight. However the campaign change is when you land on a website by clicking on promotional campaign and leave the website. After some time of just after closing the web, you land on same website by different campaign. In this situation there would be two sessions for the website.
How it works?
Time Bases Expirations: When a user land on your website and start exploring user time starts there. For example if you open a website at 09:00AM then your session expiration time on website is set at 09:31AM. But if you continue to explore the website then your expiration time will adjust accordingly. If you continue on website till 09:07AM then your expiration time will move from 09:31AM to 09:38AM.
However, if you open a website and then your switch tab without closing the website for 30 minutes then analytic would consider page sessions as expired automatically after 30 minutes. If you switch back to the website after 30 minutes then there would be two sessions by single user in Google Analytics.
Also you can see page sessions change after midnight. If you visit a website at on October 13 2018 at 11:45PM and continue to explore web till October 14 2018 00:07AM then again there would be two sessions in analytics.