Disconnect closes the user's RDP connection but leaves the session on the server. The user can connect again later and find their work where they left it.

Select one or more sessions on the User sessions tab, then:
A confirmation dialog asks Are you sure you want to disconnect all selected users?. Click Disconnect to proceed.
The Windows API call WTSDisconnectSession runs on each target server. The session state changes to Disconnected. The user sees their RDP client window close; any unsaved work in open applications is kept.
A disconnected session continues to consume memory and CPU. If you also want to free those resources, log the user off instead.