Essentially a miserable fail resulting in a win, or a clutch against all odds. It's basically pulling a complete 180, only the end result is typically extremely epic and/or significant.
Note: If you are witnessing or performing a Seth To The Rock moment it is ESSENTIAL that you scream "SETH TO THE ROCK!!!" or else it is considered just a win, not an epic win.
Football Player 1: "Our team was losing 4 -1 and there was only 45 seconds left on the clock, but then we got 2 miracle interceptions and a TD and won with 1 second left"
Football Player 2: "It was a SETH TO THE ROCK moment.
Teen 1: "Were playing search in MW2 and I was last alive vs. the the entire enemy team. I killed everyone with a riot shield and final kill was an across the map throwing knife to break the tie and win the game. It was a SETH TO THE ROCK moment.
The story of David and Goliath is an example of a SETH TO THE ROCK moment.