Hi Moin,

Resonance is the conceptual alternation of a chemical species (such as a molecule or ion) between two or more equivalent allowed structural representations differing only in the placement of electrons that aids in understanding the actual state of the species as an amalgamation of its possible structures and the usually higher-than-expected stability of the species. Refer to the definition 

Since, Resonance produces echos and vibrations in water molecule, Hence the food when kept in Microwave oven gets hot because of the resonance in water molecules which exists in any food.

Hope you understand the concept and meaning of resonance here.