From SKYbrary Wiki
Contra-rotating propellers refers to an installation in which two propellers are attached to the same engine with one mounted immediately behind the other. The two propellers contra-rotate (rotate in opposite directions around the same axis) with engine power provided via a planetary gear or spur gear reduction gear box. Use of contra-rotating propellers recovers energy lost due to the motion of the air in the slipstream of the forward propeller and allows for an increase in power without a corresponding increase in propeller diameter. It will also help counter the torque effects of a high power piston engine.
Contra-rotating propellers should not be confused with counter-rotating propellers.