Consider the three-story building below that is experiencing horizontal ground motion.
Suppose that we model the building as a two-degree-of-freedom oscillator with ground motion y(t). The side-to-side ground motion can be represented by the harmonically time-varying function y(t) shown above. This system has two degrees of freedom, having two natural frequencies and two "natural modes" of vibration.
Frequency of ground motion near the first natural frequency of the building
Frequency of ground motion near the scond natural frequency of the building