I have heard about power chords being used in a lot of metal and similar music but I have no idea what they are.
So called 'power chords' are the result of using only 1 and 5 of a chord in a key. They are neither major nor minor, lacking the defining 3 from the key. As with an E5, E and B (and optional higher E) are played.
They work well with distorted guitar sounds, as their harmonics (emphasised by the overdriven sound) don't clash too much. Putting in a 3rd produces other harmonics, which do clash.
On guitar they're easy to play, with just 2 or 3 fingers, and with drop D tuning, even easier.
A "power chord" is a combination of a root, fifth, and octave. These three pitches have a 2:3:4 frequency ratio. If the sounds are combined by any process which induces harmonic distortion, that process will add additional frequencies. If a signal contains two frequencies, harmonic distortion can add additional content at the sum or difference of those frequencies. In addition, the frequencies produced by harmonic distortion are then eligible to be added or subtracted from other frequencies to produce still more.
If a signal contains content which is concentrated almost entirely at frequencies in a 2:3:4 ratio or near-perfect multiples of those frequencies, then passing it through a distortion process will create many new frequencies, but all of them will be near-perfect multiples of half the original frequency. The net effect will be to produce a sound an octave lower than the original. While this technique is particularly popular in electric guitars, it has also been used in pipe organs for a long long time before that. The distortion effects organ pipes produce on the sound from nearby organ pipes tend to be much more subtle than those produced in electronic amplifiers, but the principle is the same.
To visually see why a power chord has this kind of effect, visualize the effect of adding together three signals with a 2:3:4 frequency ratio and clipping the result.
------______------______------______------______ root ----____----____----____----____----____----____ fifth ---___---___---___---___---___---___---___---___ octave ----____-___---_----____----____-___---_----____ combined and clipped
The top three waveforms represent the original signals (simplified to square waves for this illustration). The bottom waveform represents the sum of the first three. If the majority are high, the output will be as well. Likewise if the majority are low. Note that the output waveform has a complicated pattern, but it repeats at a frequency which is half of the original root frequency.