It means it's a perfect interval. Basically there's only one size of interval for Unison (P1), 4ths, 5ths, and Octaves (P8). The rest can be major or minor. Perfect intervals have a somewhat distinctive sound as well - more stable/neutral sounding I'd say.

Sorry, my other comment was misleading, you can also have augmented and diminished versions of
perfect intervals. Augmented intervals occur when a perfect interval is made a half step bigger and diminished are a half step smaller. Though you don't really use the terms augmented/diminished octave or augmented unison, and diminished unison would be a negative interval haha.. And you don't often see augmented/diminished 4ths on this site though you could call a tritone an augmented 4th.

Okay, so p for perfect. No worries about the misleading part, I’m sure it will make sense as time goes on. Very helpful answer, thanks Josh!