When you expand the Audio track in the Timeline, you'll notice that an audio clip is represented by a waveform. Running through the center of the waveform is a red line, called the red volume rubberband, which allows you to adjust the volume level of a clip at any point. Clicking on the red line creates a small red square, called a handle, on the rubberband, which breaks the rubberband into separate segments. The volume for a segment can be adjusted separately.
A handle is included at the beginning and ending of a rubberband segment, and you can drag the handle up or down to change the audio level. All audio clips include two handles that you can't remove—one at the beginning of the clip, and another at the end.