Before we get into Trimming, it will be helpful to be familiar with a couple of terms. So, in this section, we'll discuss A-Side and B-Side as well as the concept of HANDLE.
When you are editing (for example, using Splice and Lift, and so on), the Composer Window displays two monitors. The Source Monitor is on the left-hand side and the Record Monitor is on the right-hand side.
However, when you are Trimming shots longer or shorter (we'll talk about Slipping and Sliding later), monitors of the Composer Window change their display. The left monitor is displaying the tail frame (the last frame) of the shot on the left-hand side of the transition (the cut), while the right monitor is displaying the head frame (the first frame) of the shot on the right-hand side of the cut. However, we need a more generic and succinct way to describe them. So, we refer to the shot on the left-hand side of the transition as the A-side shot and the shot on the right-hand side, as you guessed it, the B-side shot. You may also hear people refer to the A-side shot as the outgoing shot and the B-side as the incoming shot. I'll be sticking with the terms A-side and B-side.
Handle is the material that was not edited into the Sequence, but is still available in the clip.
Handle gets revealed when you trim a Segment longer (of course, once it's revealed it's no longer Handle). On the other hand, when you trim a Segment shorter you are creating Handle. Further, Handle also comes into play whenever you apply a transition effect (for example, a dissolve). The transition effect uses the material in the Handle to do its job creating the effect.