It is an imaginary force due to incorporated effects of inertia. When a body is rotating in a circular path and the centripetal force vanishes, the body would leave the circular path.
When the centripetal force ceases to act on the body, the body leaves its circular path and continues to moves in its straight-line motion but to observer it appears that a real force has actually acted on the body and is responsible for throwing the body radially outward. This imaginary force is given a name to explain the effects on inertia to the observer who is sharing the circular motion of the body. This inertial force is called centrifugal force. Thus centrifugal force is a fictitious force which has significance only in a rotating frame of reference.