The Logged in Users module under the User Administration application lists sessions of all the users currently logged in to the system:

An administrator can view the details of any of the listed sessions by clicking on the info icon and read the entire session logs. It is possible to lock out a user's session by simply clicking on the Lock Out Session button on the Session detail page:

A user might have logged in from different browsers thus there may be a scenario where more than one session belongs to one user. When locking down sessions from the Session detail page, we are only terminating one of the sessions of the users. Also, the user can still log back in and start a new session, as they are currently not Locked out of the instance.