Asian handicap is a very popular kind of bet originating from asia , its simply reducing the betting options to just 2 thus eliminating draw.
Asian handicap is just adding more goals or advantage most times to the underdogs and placing your bet for or against the team, its a special kind of bet because it gives more options and reduces risks drastically because it eliminates draw, in a normal handicap game draw still exists so you pick among 3 options but in asian handicapping a draw simply guarantees your stake returned .
Example: man utd v chelsea .... 2(+1ah)
Asian handicap
Chelsea win ..... bet won
Chelsea draw .... bet won
Chelsea lose by 1 goal .... (void) stake refunded
Chelsea lose by 2 goals or more ... bet lost
Normal handicap
Chelsea win .... bet won
Chelsea draw ... bet won
Chelsea lose by 1 goal ... bet lost
Chelsea lose by 2 goals ... bet lost
More other asian handicap examples below