When appropriately designed, ultrathin bi-layered metamaterials can achieve near-unity absorption or serve as antireflection coatings in a narrow frequency band over a wide incidence angle range. Expanding the bandwidth is possible through using multiple resonators in a unit cell with slightly different resonance frequencies. Impedance matching by tailoring effective permittivity and permeability has phenomenologically explained the experimental observations, however, we found that the underlying physical process is a Fabry-Perot resonance which does not involve the magnetic response. Our interference model has successfully explained the properties of metamaterial absorbers and antireflection coatings such as the spacer thickness dependent reflection and anti-parallel directions of currents.