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!)
Here's some tips, but won't cover everything:
Think of 가 and 이 as being used to bring in new information, and 은/는 being used to connect what's already known to the new information.
In this sentence: 남자는 메시지가 있습니다 (The man has a message), you probably would've already known about the man with past context.
Maybe you're sitting in your office and your secretary comes in. "There's a man outside wanting to see you." They say. There would've likely been a 가 attached to the man, since it's new information that he exists.
"What does he want?" You reply.
"He (the man) has a message./남자는 메시지가 있습니다." The secretary replies. You already know about the man. He's not new information. What's new information is the message. So, 'message' is the item that will have 가 attached to it, putting more emphasis on it than the man. The man will just have 는 attached to it to attach it to the next word.
I learned this concept from Japanese, and from what I've seen so far, it seems to be the same in Korean. I never understood it when people just said "as for (item), etc. etc." when explaining は, which in Korean is 은/는, and it seems that sentence is popping up here too. I didn't even know what that meant! As for the man? How does "as for..." tell me when to use は or が (은/는 or 가/이)?
I feel like a better way to explain it would be "as for (object), which you already know about +은/는, this is what's new that exists +가/이."
So, to connect that back to the original sentence 남자는 메시지가 있습니다, it'd be like this:
"As for the man (which you already knew about, so you'd use 는 with him), he has a message (using 가 since this is new information, so it gets more emphasis)."
Of course, if you're introducing the man and the message in the same sentence, you'll just put 가/이 on whatever needs more emphasis or could be considered more important.
남자는 메시지가 있습니다. There is a man and he has a message, but the part with more attention/emphasis is the message he has.
남자가 메시지는 있습니다. (I switched 가 and 는). There is a man and he has a message, but what's getting more attention is that there's a man that has the message. 》Maybe the secretary from before came in and said "There's a message for you," making the message already known. "Who has it?" You ask. "A man has the message," replied the secretary, placing the 가 on the man because he's the new information.
Hopefully this makes more sense to anyone reading it! I know I could've used a better explanation when I first learned how this stuff worked, heheh.