When three-phase stator winding is fed from 3-phase supply, a rotating magnetic field is set up which cuts the rotor conductors. Since the rotor circuit is closed, currents start flowing in the rotor conductors. Now, rotor conductors are carrying currents and are in the magnetic field. Therefore, mechanical force acts on the rotor, tending to move it in the same direction as the stator field.
The fact that the rotor is urged to follow the stator field (i.e. rotor moves in the direction of stator field) can be explained by Lenz’s law. According to Lenz’s law, the direction of rotor currents will be such so as to oppose the cause producing them. Now, the cause producing the rotor currents is the relative speed between the rotating field and stationary rotor. Hence to reduce this relative speed, the rotor starts running in the same direction as the stator field and tries to catch it.