The frequency of an object is its number of vibrations per second as measured in Hertz (Hz), whereby 1 Hz = 1 wave per second.
The natural frequency of a physical object is the frequency at which it vibrates after excitation. The natural frequency of a building will depend, among other things, on its method of construction, the materials used in it and its height. Standard Swiss buildings two to five floors high generally have a natural frequency of 5 Hertz.
When external causes, like shockwaves of an earthquake, 'excite' an object, like a building, at its natural frequency or thereabouts, the resulting vibrations are particularly strong, i.e. have a high amplitude (see the question "What is amplitude?"). This phenomenon is referred to as resonance. The greater the resonance between a building and the vibrations caused by an earthquake, the greater the risk that the building will suffer damage. Consequently, when earthquake-resistant buildings are constructed, care is taken to ensure that their natural frequency differs from the shockwaves caused by a potential earthquake.