Each concurrent-user license has a maximum number of users who can access your account at the same time. The system counts each active user to determine whether you have hit your user limit.
After 15 minutes of no activity, a concurrent user is considered to have abandoned the session and that seat becomes available for another user.
If a user reengages with the website after 15 minutes, that would be considered a new session, so they can get access as long as a seat is available within the concurrent-user count limits for the site license.