What does it mean when you walk by a guy and he has a erection?

Answer 1

"Boner" is slang for an erection of the penis. This usually happens when he either has to pee, or is turned-on sexually. Since you can't make a guy have to pee, its obvious that it means the latter. That means you did something that he finds attractive. That's a good thing, just so you know, means you're attractive

Answer 2

All men, gay or straight, at any age will check out another guy given the chance. Everyone is worried about how they measure up in the group they happen to be in. An individual may be looking because it is unexpected or a particularly large or small one. He may be interested or not. A follow up question is why do you have one where another guy can see it ? All guys look.

Answer 3

It means he is physically attracted to you.