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