What's meaning of the 'Rep.'?
Edit: As Justin points out in comments, that’s what Rep. means in this particular example, but in other contexts it can mean many other things too.
It should be pointed out that this is actually ambiguous in this case. It could well stand for Representative, as the gentleman in question was in fact a member of the US House of Representatives at the time.
However, it is also a common abbreviation for Republican, which is the political party he is a member of.
I'd guess the poster was aware of this ambiguity, and was fine with it. Space is so limited in a tweet that doing double-duty with a word like this is often considered a good thing.