I guess you'e talking about the gyroscopic turning tendency for single engine airplanes among other things. The basic physics has it a force placed on a gyroscopic disc such as that formed by a rotating prop, causes a reaction to occur at a point 90 deg. behind the original point in the opposing direction. "Behind" here means going backwards in the circle of rotation.

So, you are sitting in an airplane behind a prop that spins clockwise from where you sit. You place a force on the top of the prop disc somehow- we'll get to the how later- and a force is felt 90 degrees to the left side in the opposite direction (towards you). This reaction causes the prop disc and hence the airplane, to want to turn to the left. It's called left-turning tendency.

So, how did you apply that original force at the top of the prop disc? The usual and most important way is through a sudden raising of the tail on a tailwheel airplane using the stick. There are other ways, but that is the most critical one. It can lead to loss of control during takeoff roll in larger airplanes with big engines.