You can activate a clip to edit the keys it contains from the Time Slider. Also, when a clip is active, all the keys you set for the channels in the active clip are automatically placed in that clip. See Active/inactive clips and Key into a clip.
When a clip is active (Activate Keys in on), Maya ignores clip settings such as Weight, Offset, and so on.
If you set new keys on the clip that extend past the end of the active clip’s duration, the keys play while the clip is activated, but won’t be included in the deactivated clip unless you adjust its Source In/Source Out channel values or modify the source clip’s duration in the Channel Box. For information on editing a clip’s keys in the Time Slider, see Edit a clip’s animation curves.
To activate or deactivate a clip
-click the clip and select Activate Keys from the menu that appears. See Activate Keys.
When a clip is active, a 
appears beside the Activate Keys option in the Trax context-sensitive menu for the clip. When a clip is disabled, the clip appears purple.