define eddy currents and explain there formation?? why does the eddy currents stop the motion of the coil??

When a conductor of is moved in a magnetic field or is kept in a changing magnetic field, electric current is induced throughout the volume of the conductor and flows in closed loops. The current can be thought as whirlpools or eddy that forms in the rivers or lakes.

According to Faraday’s law changing magnetic field produces electric field in a conductor.  Electric field in a conductor gives rise to electric current according to ohms law. Or in other words the induced field gives rise to emf which lets current flow in closed loops.