Imagine an airplane with the motor running, but the prop held such that it can't move. The plane will be spinning in the opposite direction that the prop was originally turning. Normally the airplane has more mass than the prop, so it wins the battle and the prop turns, but that prop is always trying to force the airplane in the opposite direction.

When viewed from the rear, propellers normally turn clockwise, to the right and down. This makes the plane want to roll to the left and up, opposite the rotation. This is the main reason why you will see right thrust and down thrust on tractor (motor in the front) airplanes. On a pusher (motor in the back) airplane, the effect is somewhat opposite: The plane still wants to roll left, but the nose wants to go down, so you need to counter with up thrust.