“He's a guy from my apartment complex.”
You run into someone at a shopping mall who lives near you. You're with your friend, who asks how you know this person. You explain.
He's a guy from my apartment complex.
A "guy" is a man. It's a casual word.
You use this word instead of "man" when you're talking to your friends or in a casual situation like at a party. The person you're talking about can either be someone that you know, or someone who you don't know but don't have high respect for.
For a stranger who seems older and more respectable, "man" or even "gentleman" are more polite.
Note that you can use the plural form, "guys", when you're talking to a group of men or women:
But "guy" (singular) always refers to a man.
Sometimes a single company builds and manages a group of apartment buildings. The buildings might have some shared facilities like a pool, laundry room, or fitness room. This group of apartment buildings is called an "apartment complex".
Other similar examples of "complexes" include:
an office complex
a warehouse complex