To be reliant is to depend on someone or something. When you're reliant on a person, you need that person.
There are a lot ways people and things can be reliant. Kids are reliant on their parents for food and shelter. We're all reliant on the police to keep us safe, but the police are reliant on the government for funding. Being reliant is about being dependent. If you can do without something, you're not reliant on it. When you feel like you don't need other people, you can say you're self-reliant.