What is the meanings of these sentences?
1.What a riot!
2.You are a good sport.(being a good sport)
"What a riot" can be literal (there is a crowd rioting outside! Look out!) or just a saying suggesting the events were hilariously funny.
Being a good sport means you can take verbal abuse or jokes about you in a good-hearted, light, or laughing spirit. It also means you play by the rules in a game of sports, although in that case it's more usual to say, "he is a good sportsman." But now, with political correctness, the -man part is disappearing.