Definition of light
Light means "light". In poker, you can make a 3bet light, a call light, a 4bet light, etc. What is meant by "light" is "with a weak hand". If you make a 3bet with KK, you are in value, you are making a strong 3bet. If you make a 3bet with J9 suited, you are making a light 3bet. There can be many reasons to make a "light" play. You can call light because you suspect your opponent is bluffing a lot, you can 3bet light because your opponent often folds in 3bets, you can 4bet light because you have a tight image and your opponents will give you credit, etc.
Two types of light games
Pros and beginners will make light games. The difference is that each will have its reason. If a beginner calls too much on the rivers (call light), it may be because he is a calling station, that is, a curious player who wants to go to every possible cull, even if he loses most of the time. A pro who makes a light call will have a good reason. If he suspects his opponent is bluffing at a high frequency, he will make a light call. When you make a "light" play make sure you have a good explanation for thinking like a pro and not a beginner.