Clip handles are extra footage before the clip's in-point and after the clip's out-point that you can use to manipulate a transition.
If the source media does not contain enough frames, clip handles may not be present as there is no extra footage to add the transition. When you add the transition, Premiere Pro warns you that there is ‘Insufficient Media’ and that the transition will contain repeated frames. Premiere Pro automatically creates its Clip Handles by repeating the end frames to form a freeze frame of the clip. The transition appears in the Timeline panel with diagonal warning bars through it.