The three-bet light is an online poker term that is used to describe three-betting with non-premium hands as a form of bluff to get folds from better hands.
Three betting “light” with a wider range of hands then you would normally is a skill you will have to get used to especially as you begin to progress through the various stakes online and live.
The problem with only three betting your premium hands in a predictable manner against observant opponents is that they can easily put you on a narrow range of hands, something like QQ+, AK, and so they won’t get paid off with your big hands, and you will be easily to read and play against.
With that being said, 3-betting light at the lowest stakes games isn’t a necessity. The difficulty with 3-betting light in micro-stakes games is that a lot of opponents are going to call you off with anything, which defeats the whole purpose of trying to get them to fold pre-flop. And then you find yourself in a position where you’ve completely missed the flop and try and bluff a calling station off a hand when you should have just folded pre-flop.