Trimming is cutting off the beginning or end of a clip.
You can trim a clip before it's in the Timeline or after, but it's better to do it in the Timeline, so that you can see how the transition looks with the adjacent clip.
To begin trimming, choose the Selection tool (the black arrow) from the upper left corner of the Timeline, then select the video clip that you want to trim.
Put the Playhead over the clip you want to trim (it's that triangle with a long, thin stem that runs over all the tracks). That lets you see the current frame in the Monitor window.
Position the cursor at the beginning or end of the clip until it changes to the Trim tool (looks like a red bracket). Click and drag to trim the video down to the desired frame or length.
If the clip has synced audio and video tracks, the Trim tool will trim both the tracks. To trim only one of these tracks, click the toggle sync mode button at the bottom of the timeline to temporarily unsync the tracks (and make sure you resync after you're done).