Alright so endeavour takes your opponent down to the same amount of HP as you and is a normal type move.
The most commonly seen uses of endeavour are on FEAR strategies or something like mamoswine.
So basically on a level 1 Pokemon like Rattata, Endeavor is scary. Since Rattata is level 1, when it gets hit by a single attack it gos down to 1 hp assuming you have Focus Sash - then it uses endeavour which brings down the opponent to 1 hp. Then quick attack and the opponent dies.
Similarly on Aron, it uses a combination of shell bell, sturdy and sandstorm to sweep through teams without steel/rock/ground/magic guard/overcoat Pokemon. Mamoswine does it through Focus Sash and Ice Shard which is a priority finisher.