First he caught a touchdown that put the Giants ahead in the fourth quarter. The Patriots scored after that, so the Giants had one drive to win the game. On 3rd and 5, Giants QB Eli Manning was almost sacked by 2+ Patriot defenders. Somehow he got away from all of them and threw the ball downfield. David Tyree caught the ball with his arm and his helmet, as his other arm was being held by Rodney Harrison.
That was the play of the game, and it has many names such as Helmet Catch, Double Miracle, David and Eliath, The Great Escape, The Great Helmet Escape Catch, etc.
This catch probably saved the game for the Giants, as Manning went on to throw a game-winning touchdown to WR Plaxico Burress, ending the Patriots' perfect season as they were 18-0 at the time. Now they get to be 18-1. Losers.
-Bob Papa (yes, that is the real name of a broadcaster.)