YAC means Yards After Catch. So to get this trophy, you must have a player on your team catch any number of passes and have them gain yards after they make a catch.
Start up a quick play game on Rookie difficulty. I used the Detroit Lions so I could use Calvin Johnson (wide receiver). He makes this trophy almost too easy. But you can use any team that you like. Just make sure you throw to the same receiver enough to get him over 100 YAC. You can check your progress in the in-game stats.
The trophy will pop at the end of the game!