A 300 yard game is when an offensive player rushes 300 yards, passes 300 yards, or catches 300 yards worth of passes, in a single game. It is a notable feat because no player has ever rushed for 300 yards, and only 4 players have ever racked up 300+ receiving yards.
Dan Marino and Peyton Manning are currently tied for the most 300 yard passing games in their career (63). Drew Brees had 13 such 300 yard passing games in 2011, and he holds the single-season record. Coincidentally, Drew Brees also holds the record for most consecutive 300 yard passing games, made in the same season (7). Adrian Peterson has gotten the closest to breaking the 300 yard rushing threshold, having rushed for 296 yards for the Vikings when they played the Chargers on 11/4/07. Jamal Lewis also got close with 295 yards rushing for the Ravens when they played the Browns on 9/14/03.