It's a possibility determinant. Example What about your friend? He might come 5-10% He may come 15-20% He could come 25-40% He should come around 50% He will come around 75% He must come 100%

Might is past tenses of May and Could is past tenses of Can. May/ might: when you want to ask for a permission (May I make a phone call) or a offer (May I help you?) Can/could: You can use it the same as May. Or, When you describe ability (I can speak English) or when you talk about something is possible or impossible (I can't call her because her phone is always engaged) American style is usually Might/ Could when you talk about future possibilities. (I think the price could be increase in future).

Can you help me