Log off ends the user's session and discards it. The user has to sign in again to start a fresh session. Any unsaved work is lost.

Select one or more sessions, then:
A confirmation dialog asks Are you sure you want to log off all selected users?. Click Log off to proceed.
The Windows API call WTSLogoffSession runs on each target server. The session ends. Applications in the session receive their normal session-end notifications; whatever they had unsaved is gone unless they autosave. If the user is still connected when you log them off, they see their RDP window close.
Tell the user before you log them off if you can. Use Send a message to give them a few minutes to save their work.
WTSLogoffSession API and end the session. Pick Log off for normal cleanup of a finished user and Reset for sessions that have to go now (see Resetting a session). The user-facing effect is the same; the labels exist to keep the two intents separate in muscle memory and in the audit log.