alternating current (AC)
Direct current (DC) flows one way, like a river. Alternating current (AC) sloshes back and forth, like the tide, reversing direction many times a second. The wall socket delivers AC; a battery delivers DC.
In AC the voltage and current vary with time, usually as a sine wave that swings positive then negative around zero. Mains electricity alternates 50 or 60 times a second (50 or 60 hertz) depending on the country. Because the current periodically reverses, AC can be stepped up and down in voltage by transformers with almost no loss, which is why the grid uses it for long-distance transmission.
Nearly all signals (audio, radio, sensor outputs) are AC riding on, or instead of, a DC level, so understanding AC is the gateway to everything beyond simple DC circuits. Honest note: AC usually means a sine wave, but any repeating back-and-forth such as a square wave or music is AC too; the sine just happens to be the simplest building block all the others are made from.
A 9 V battery is DC: connect a meter either way and it reads +9 or -9 steadily. A wall outlet is AC: its voltage rises and falls and reverses 50 or 60 times each second, averaging to zero.
DC holds steady; AC reverses.
The label AC describes the waveform, not the wire. The same wire can carry a DC level with an AC signal riding on top, which is exactly how audio coupling works.