Induction Heating is a process where a metal object, magnetic or non-magnetic, is placed in a varying magnetic field, technically known as Eddy Currents. Imagine you have a round copper coil and then you pass an alternating current through this coil. That would generate a magnetic field within the coil.
Then imagine you interrupt this field by inserting a piece of metal, inside of this coil. The magnetic field would begin to wrap itself around the workpiece at a very high rate. How high a rate depends on the frequency of the applied RF field. So, for instance, if the RF frequency is 50kHz, then the field will wrap itself around the workpiece 50 times per second.