Power chords are fun and simple. I use them sometimes as an alternative to bar chords.
There are three strings being played in a bar chord. Those three strings are the E, A, and D strings (4, 5, & 6).
Bar chords are hard to explain to in words, so I mad a tab chart to show you how to play power chords.
E Power Chord F Power Chord
Those are only examples of two power chords on the low E string. Keep moving up fret by fret and there you go. You can also do the same stuff on the A string with the same three finger pattern. But once you get to the D string, things get complicated. Check out this helpful chart below.