The cooker generates an electromagmetic field. This field "projects" out into the area above the cooktop. When cookware containing iron (ferrous cookware) is placed inside this field, this field makes the electrons in the cookware move around rapidly.
The friction associated with this electron movement heats up the metal in the cookware.
Non-ferrous cookware does not produce the same result.
This is a simplistic view of what happens and should be good enough to kind of get a picture of what is going on but is not the complete story, from a technical standpoint.
Is that enough or do you want to get into the gory details?