A nice guy is someone who tries to be kind and considerate to other people. They like to help others, be polite and respectful, and generally want people around them to feel comfortable and happy. Think of them like they're your best friend, always wanting you to have a good time and feel good!
Sometimes, we might hear someone say that someone is a "nice guy" when they're trying to make it seem like that person isn't being genuine. They might be trying to say that the person is pretending to be nice, when really they have other intentions. But for the most part, being a nice guy is a really good quality to have. It's important to treat others with kindness and respect.