That was the final score in the final set, won by John Isner over Nicolas Mahut at Wimbledon. In the final set of a match the winner has to beat the other by at least 2 games, so as in baseball matches can (theoretically) go on indefinitely. In practice even final sets that go into “extra innings” tend to end quickly. The previous record for number of games in a final set was 1/3 as many. For some commentary on the odds related to this extraordinary match check out:
Wikipedia has a list of Longest tennis match records.