When it first came out, the title Groundhog Day simply referred to the fact that this film takes place on Groundhog Day. And sure, the name is appropriate because the whole tradition of Groundhog Day implies that the flow of time is so fragile that it can be determined by a cute rodent.
But the real meaning of this movie's title has really appeared since the movie came out.
Since the release of Groundhog Day, the term "Groundhog Day" has entered common language to signify an unpleasant situation that keeps repeating itself. The spread of this term just goes to show how unique the plot of this movie is. You'd think that someone would have come up with this idea before, but Groundhog Day looks like it's really staked a claim on this concept of reliving the same experience over and over.