3-betting light means you are using a bluff/semi bluff
Typical Candidates: A5s-A3s, JTs-87s, KQo-QJo
Basically anything that you would fold facing a 4-bet, but typically hands that will 'play well postflop' meaning they flop top pair or strong draws often
3-betting heavy means you have a value hand (typically one you are willing to go all-in with)
Typical Candidates: AA, KK, QQ, AK, AKs, JJ
Secondary Candidates: TT, 99, AQ, AQs
Tertiary Candidates: 88, 77, AJ, AJs, KQ, KQs
Hope that helps!