Centrifugal force is the force that is equal in magnitude but opposite in direction to the centripetal force, which is the force directed toward the center of a rotating or revolving mass.

Think of it this way. Let's say you have a yo-yo and let it unwind. Then you start swinging it around your head in a circle parallel to the ground. The force that keeps the yo-yo in its circular path is the centripetal force. Without it, the yo-yo would not continue in its circular path but would fly off in a straight line, which it is inclined to do. The tension in the string, the force that makes the string taut, is the centrifugal force.