A power chord is the common term for what is traditionally known as an open chord. An open chord can be a two-note chord, at the interval of a perfect 5th. It is more commonly a three note chord, consisting of a root note and intervals of a perfect 5th and an octave.
An open chord is so-called because it is "open" and ambiguous. Heard on its own, it is vague as to whether it is a major or a minor chord. The third of the chord, which defines its tonality, is omitted. Open chords feature a lot in rock music, and their use in combination with driving off-beat rhythms give the music a sense of drive.
It is not incorrect to use the third of the chord in the accompaniment when playing piano covers of high-octane rock songs, but the bluntness of the open chord, as opposed to major and minor chords, is best suited to capture the full flavour and rawness of such rock songs when they are covered on the piano .
A beginner could start playing the simple open chords at an interval of a fifth on the piano, using the thumb and little finger. Intermediate and higher learners might prefer the fuller three-note versions - using the little finger, index finger and thumb - and combining these with the rhythmic patterns inherent in the song.
Experience the different impact on the piano with open chords! Even if rock or metal songs are not usually what you would listen to, open chords would be a useful addition to your piano technique.