A capacitor smooths out a pulsed direct current into a "less jerky" direct current.
Work Step by Step
Recall that a capacitor stores electric charge (think "capacity" - a larger capacitor stores more charge at a given voltage). A jerky dc current is like spurts of water coming through a hose. If we direct that jerky current into a capacitor (think "large water tank"), the output is much smoother than the input (think about the water that drains from the water tank while the jerky current enters it). This is discussed on pages 437-438, and shown in Figure 23.11bc.