FEAR strategy is probably not a great choice for a normal playthrough of the game.
It means: Focus Sash, Endeavor, Quick Attack, Rattata. The pokemon doesn't have to be a Rattata, but Rattata makes a good choice.
You'd make sure that your level 1 Rattata (or other pokemon who can learn this moveset) has a Focus Sash equipped. On your first turn of battle, you use Endeavor. Since you're level 1, your opponent is almost certainly going to out-speed you and one-shot you, but the Focus Sash keeps you alive with 1HP. Then, your Endeavor will bring your opponent to 1HP, since it sets your opponent's HP equal to yours.
On your second turn, you use Quick Attack, which allows you to attack first and deals at least 1 damage (because no damaging attack can deal less than 1 damage), which is enough to kill your opponent's pokemon.
It's a one-time-use gimmick to kill a single opponent's pokemon. The focus sash breaks afterwards, and even if you have another one, you'd still need to visit a poke-center or use items to full heal so that you can make use of the Focus Sash again.
More information on Bulbapedia.