The word "gentleman" just means "man" or "guy", but it's a more formal word. You can use it when you're talking about a person who you don't know very well, in a situation where you're trying to seem polite and respectful. Here are some examples of situations in which you'd refer to a man as a "gentleman":
- You're describing a customer at the store you work at to your boss.
- You're explaining to the police how a fight got started.
- You're giving a speech to a large group of people at a business conference, and you want to talk about a man who's sitting near you.