This is hard to answer. A friend might say this to convince you to try something new (and possibly dangerous). "Come on. What's the worst that could happen?" What is the most horrible thing you can think of, that might result if you do something? You might also say it to yourself, if you were trying to get up the courage to talk to someone you find attractive. "What's the worst that could happen?" You might be rejected. Can you deal with that? Yes? Then go for it!