As Sompuperoo said, it's an abbreviation that gives the political party and state of the congressperson. It's very common to use the two-letter abbreviation for the state, as in R-TX (Republican from Texas) or D-NY (Democrat from New York). This kind of abbreviation is normally only used after a person's name. You wouldn't say something like "He's D-FL" unless you were playing with words.
On political blogs you'll often see a similar abbreviation used to refer to numbered congressional districts within a state, such as TX-18 (the eighteenth district of Texas) or NY-04 (the fourth district of New York).
I've only seen this sort of abbreviation used in U.S. politics. As far as I can tell from a quick Googling, the UK, Australia, and Canada prefer to spell out where their MPs are from and what party they belong to. (Commonwealthers, please correct me if I'm wrong.)