The frequency of a sound is the rate at which wave crests reach a given point. Sound waves propagate outward from their source, as a series of compressions and rarefactions of the surrounding air. The peak of each cycle can be plotted as the crest of a wave, as can the low point or trough. The frequency with which these crests and troughs arrive gives sound its pitch.
The Physics Classroom explains that the distance between the peak of one sound wave and another is known as the sound's period. A low period corresponds to a short distance between the wave crests, while a long distance between the wave crests corresponds to a long period. The period of a sound is thus the inverse of its frequency.
Low-period tones have a short distance between crests and, therefore, the waves are reaching their observer rapidly and with a high frequency. This raises the pitch of the tone. Low-frequency sound waves are stretched out, with a long period between them, so fewer wave crests reach the observer in a given length of time. Sound frequencies are expressed in units of vibrations per second, or Hertz, where 1 wave crest per second gives a frequency of 1 Hz.