Translation:The man laughs.
I'm still trying to figure this out as well, but what I've gathered from experience so far is that using 가 would mean "The man [subject] laughs", or possibly "The men [plural subject] laugh".
Using 는 in the same sentence, depending on context, could mean any of the following: "As for the man [topic], he laughs" (anglicized: "The man laughs"); or "As for the men [plural topic], they laugh" ("The men laugh"); or "As for men [general topic], they laugh" ("Men laugh"); or "As for a man [indefinite], he laughs" ("A man laughs"); or "As for some men [indefinite plural], he laughs" ("Some men laugh").
Tl;dr: If you see 가 , you can safely(?) deduce that the word is the subject. 는 on the other hand is much more complex and can mean a bunch of different things depending on context.
(Somebody correct me if I'm wrong though!)