You should basically always slow-play when you flop quads or better. Furthermore, you also want to routinely slow play strong hands versus loose and aggressive players because they like to bluff too much.

A slow roll is when a player delays showing a winning hand at showdown or delays calling a bet/raise with a very strong hand before showdown. The goal of a slow roll in poker is to make the opponent think they are about to win when they really aren't. This is often seen as poor etiquette.