From SKYbrary Wiki
Counter-rotating propellers are installed on some twin and multi-engine, fixed wing aircraft. They are called counter-rotating propellers because the propeller(s) on one wing turn(s) in the opposite direction to the one(s) on the other wing.
On most twin or multi-engine propeller driven aircraft, the propellers all turn in the same direction, usually clockwise when viewed from the rear of the aircraft. In a counter-rotating installation, the propellers on the right wing turn counter-clockwise while those on the left wing turn clockwise. The principle advantage of counter-rotation is to balance propeller torque effects thus eliminating any problems associated with a Critical Engine.
Counter-rotating should not be confused with contra-rotating which refers to two propellers on the same engine which spin in opposite directions.