A power chord is a chord stripped of all its parts except the bare essentials:
E maj (02210) -> E5 (02xxxx)...I hope you know what the numbers mean, it's kinda hard to explain this to someone just starting...but don't get discouraged, you'll get it. Power chords are used ALOT in metal.
PS: the power chord is listed as E5 because the 2nd note (the '2') is a 5th and don't worry, it's not annoying.