Welcome to the forum ukphil :thumb:
It usually means adding the hops to the hot wort after the boil. It's a way of extracting volatile/aroma oils from the hops without boiling them off. You can add them when you want really as long as it's hot enough to kill any potential nasties, I add mine at 80 deg c for 10 mins.
Length of time and temp are personal choices :thumb: