The numbers in the circles illustrate comfortable fingering for the fretting hand, remember that the numbering begins from the index finger (1) to the pinky (4).
x - means this string shouldn't be played (avoid picking this string).
o - means open string, you pick this string but do not fret it, also watch that the fingers do not touch it somewhere on the fingerboard, otherwise it will sound muffled.
If you don't understand how to read guitar chord diagrams, here is the detailed tutorial.
You can see that each power chord has the same simple structure:
• the base root note located on the lower string,
• the fifth on the next string, shifted 2 frets towards the bridge,
• another root note that stays at the same fret as the "fifth" on the nearby string,
What actually changes from chord to chord is their locations on the fingerboard.
So you can easily figure out any power chord without looking into the chart if you know where the base root note stays.
Here are the notes on the A and E lower string:
Note that power chords on the higher strings have a slightly different structure.
The power chords can be a great starting point for composing rock and metal guitar riffs.
You can just play around different power chords and sooner or later you'll notice that some progressions of chords sound well.
As an example I took this progression:
C5 → → E5 → → C5 → → D5 → E5