I'll just leave this here for you:
A major chord is made up of three notes from the Major Scale - the first note, call the root, the third note of the scale, and the fifth note of the scale. To make a minor chord, you flatten the third by a semitone (ie, by one fret.) Power chords are just the root note and the fifth.
One way to play this song is to use a rhythm pattern where you play, on the first beat, the root note. So, on the one you just play the open A string (ignoring the capo.) This is what I called 'bass' above. Then, on beat two, you strum the treble strings. So, 'Bass two' is the first two beats of the bar. You could play this with a pick, but I tend to use my thumb on the fifth string and my index finger to strum down.
Now, the notes in A major are A, C#, E. These are the first, third and fifth notes of the A major scale. You notice that the fifth is E. On beat three of the bar, I play the open E string. So now we have Bass Two Fifth. And to end, I strum up on the 'and' after beat three, where I played the fifth on the open E string, and strum down again on beat four. This is what I meant by 'bas two fifth and four.'
Similar rhythms are used in Johnny Cash songs (boom chicka), where boom is the bass or the fifth and chicka is down and up on the treble strings. The A Team by Ed Sheeran can also have a related pattern.