This is pretty self explanatory, defeat Yogg-Saron in 7 minutes or less. This is of course, easier than it sounds!
Some tips:

1. Make sure your best DPS goes into the portal during Phase 2. It would be helpful to have Heroism/Bloodlust for the portal group, if possible.
2. Make sure nobody in the raid dies (you'll need everyone if you really want to beat out the timer)!
3. It helps if you have a solid group that works well together and is really on top of their game. You may be able to pug the best geared people on your server, but if you don't communicate, coordinate correctly and in a timely manner you will not get this achievement