Where did you see it mentioned? There are a lot of variations on push-ups, but I've never heard this particular term (it could be someone's name for a move that we DO know, but don't recognize).
One possibility: a "pyramid" push-up is a term some use for a push-up done with the hands in a triangle position (thumbs touching and pointer fingers touching, so when you look down, you see a triangle or pyramid). I'm not sure what the "core" part of the description would add, if anything, to this move.