How did this happen?
You win a "game" by scoring at least four points and being up by at least two points. This could go on forever, but since it's the same person serving throughout the game, it probably won't.
You win a "set" by winning at least six games and being up by at least two games. This could go on forever, but in some situations at 6-6 a special tiebreak game kicks in. You win this one by scoring at least seven points and being up by at least two points. This could also take a long time, but it's normally not a ridiculous amount of time. Well, I don't really know; I don't follow tennis. Anyway, a set that goes to this is always scored 7-6 (n) and n is the loser's number of points in the tiebreak game (because you can easily compute the winner's points from that - the winner scored 7 or n+2, whichever is more).
You win a "match" (the whole kit and caboodle) by winning 2 out of 3 or 3 out of 5 sets. Men's majors like Wimbledon are best-of-five, and Wimbledon and a few other majors forego the special tiebreak game in a fifth set.
So if you're tied 6-6 in a fifth set you just keep playing games again and again until someone gets up by two games - then they get this set and therefore the match.
And with these two guys, nobody pulled ahead by two games. Their fifth set alone has gone on for 424 minutes (about 7 hours) so far, and they are presently tied at 59 games apiece. Someone will have to have won at least 61 games to end the match - of course, more and possibly lots more if the other guy keeps up.
Now you and I can finally understand this line score. The first numbers are for the Frenchman Mahut, who is said to be "leading" - I guess he has more points (449 to Isner's 428), but that is kind of meaningless. I might not understand that properly. But here's the score so far:
4-6 6-3 7-6(7) 6-7(3) 59-59