8.5.2 Session Context for Temporary Values
A user session is the span of time during which a particular end user interacts with your web application.
It begins when the user first navigates to a page and logs in, and ends when they sign out or remain idle long enough to trigger a session timeout. At any moment, multiple users may be using your app, each working on different tasks. To streamline the experience for all concurrent users, APEX assigns a unique session ID to each one. This gives the context for each user’s interaction with the app and for the session state the APEX engine manages for each user session.
Parent topic: Understanding Session State Management