 You may have heard the terms AC and DC component used to describe an electrical signal. The DC or direct current component is the aspect of the signal which seemingly doesn't change, acting as an overall offset. The DC component continuously flows in one direction, a fixed positive or negative voltage. And the AC, alternating current component, is all other changes which occur around that offset. The AC component continually switches between increasing and decreasing voltage. So here's an AC signal, a 1 volt peak-to-peak sine wave, which now has a non-zero DC component of 3 volts. By changing the scope's coupling setting, we can switch in a blocking capacitor which only allows the AC component to pass, so we can focus on that sine wave.