There are many electromagnetic structures used in microwave engineering and EM devices. For example, patch antennas, metamaterials made from unit cells, etc.
When they design structures like patch antennas, split-ring resonators in metamaterials, and many other similar microwave structures, they want the device to resonate at a required frequency, and somehow, the operation of the device depends on it being resonant at a given frequency.
What is the meaning of this resonance exactly? Why, for example a patch antenna or a split-ring resonator needs to resonate to function properly?