FEAR stands for Focus Sash Endeavor (Quick Attack) Rattata
The point of FEAR is to have a low leveled Pokemon (usually level one) and make it hold Focus Sash. Then when your opponent uses a powerful move that would usually KO you, the focus sash sves you, and then you use Endeavor, which takes the opponent to 1 HP. Then you use Quick Attack to KO it. Of course, with Rattata and other Pokemon without Sturdy, you would only be able to use it once.
If you have Aron, or another Pokemon with Sturdy, then you can make it hold a Shell Bell, so instead of only it working once, it can work multiple times.
Check out this link for more info.