If someone tells me to play a D power chord a fifth "below", what am I to play and why?
A power 'chord' consists of a root note, the note a P5 above it, and sometimes the note an octave above that root. As in D - A - (D).
To play 'a fifth below' would be to use the same notes, but as an inversion. Probably called a second inversion, but who knows, as there's no 3rd there,(and with only two notes to play with, could there ever be anything more than a 1st inversion..?) but using theory, that inversion will have 5 underneath. So the notes would be A lower than the next D.
What does it mean to play an interval “below”?
You need the specific interval and the reference tone. Like: play a perfect fifth below
A. It's not clear if the intention is to play both tones together, but I would expect that. Someone could say I'll play this riff and then add you play it an octave below to double the part.
...If someone tells me to play a D power chord a fifth "below"
...they are being vague, or just redundant.
Actually, they don't need to make any reference to what that chord is below. They just need to say what octave or, if this is guitar, what string to play it on. Saying play a D power chord a fifth below, means first of all it would need to be below an
A to make sense and then you would play a
D with an
A above it. But you would do the same thing if simply told play a D power chord. You would still play a
D with an
If they meant for you to play
A below the
D, you could probably just call that an inverted power chord. Play power chords in fourths would seem to convey the same thing.
...play a D power chord [with the] fifth "below" [the root]
...something like that fills in the blanks.