Cogging or magnetic locking refers to the phenomenon in which the rotor of the induction motor gets magnetically locked to the stator and the motor refuses to start. Cogging is caused by a wrong choice of rotor 
and stator slots. If the number of stator slots is equal to the number of rotor slots, the reluctance is minimum when the stator and rotor teeth face each other and the motor refuses to start. This is overcome by making the number of rotor slots unequal to the number or stator slots.
If certain harmonics are present in the supply, the slot frequencies can coincide with the harmonic frequencies and cogging can result.
To prevent cogging, the rotors are skewed with respect to the stator such that each rotor slots is facing more than one stator slot at any given time.