All matter is made up of atoms and molecules.
Microwave energy will pass through electrically neutral compounds. Among these neutral materials are glass, paper, china, and some plastics. Absorption occurs when matter is not electrically neutral and the moleccules act like magnets attempting to lineup with an electrical field that is applied. While this electric field is changing millions of times each second, other forces act to slow the molecules down. The energy of the microwaves trying to overcome these forces is converted to heat.