Guy is a super common word in English, especially in casual conversations. It's got a bunch of meanings depending on how you use it.
1. Basically means a man or boy: "That guy over there is my brother."
2. Can refer to people in general: "Hey guys, what's up?"
3. In slang, it's used for both genders: "She's a cool guy."
4. Sometimes means boyfriend: "She's bringing her guy to the party."
5. In "the guy", it means the right person: "He's the guy for the job."