It's supposed to mean: "I agree" or "I have this bug too" or "Yes, this should be implemented"

Typically, I think it would mean "I would like this issue/request to be fixed/implemented."

I think this bug is an awesome thing and shouldn't be fixed? Something like that would actually be surprisingly useful.

But seriously, it likely means the person up voting the issue has also had the same problem and wants it fixed as quickly as possible.

“I have the same problem” or “I’m able to reproduce” maybe?

You want the contributers to focus on this issue.