More than one person can log in to the router and modify the configuration at one time; you want to prevent someone from overwriting your configuration changes.
Use the following version of the configure command to enter configuration mode:
aviva@router1> configure exclusive warning: uncommitted changes will be discarded on exit Entering configuration mode  aviva@router1#
Because more than one person can log in to the router at the same time, several people may be modifying the configuration simultaneously. You will know that another person is editing the configuration when you enter configuration mode:
aviva@router1> configure Entering configuration mode Users currently editing the configuration: mike terminal p3 (pid 1088) on since 2005-02-30 19:47:58 PST, idle 00:00:44  aviva@router1#
Here you can see that mike is also logged in, is working in configuration mode, and has not typed anything for 44 seconds. However, if someone enters configuration mode after you do, the CLI does not display any message, so you will not be notified. Instead, you need to check:
 aviva@router1# status Users currently editing the configuration: aviva terminal p0 (pid 3358) on since 2005-06-09 11:22:18 PDT  mike terminal p1 (pid 3768) on since 2005-06-09 11:48:31 PDT 
If you need to ensure that no one else can change the configuration while you are modifying it, use the configure exclusive command to enter configuration mode. With this option, no other users can change the configuration as long as you are in configuration mode. If you do not commit the changes you make, they are lost when you exit from configuration mode.
If another user has locked the configuration, you can forcibly log him out:
aviva@router1> request system logout user mike
You get the username from the message displayed when you enter configuration mode or from the status command.
Router Configuration and File Management
Basic Router Security and Access Control
Routing Policy and Firewall Filters