Vocal fry, also known as the pulse register, is a low-pitched vocal quality so called because the sound is similar to food cooking in a hot frying pan. It is characterized by intermittent "pops" of sound and is produced by the vocal folds opening and closing irregularly and unrhythmically.
The sound of the human voice, no matter what the pitch, is essentially a series of small puffs of air, many per second, separated by the closure (or partial closure) of the vocal folds between each puff. However, if there are enough puffs of air per second, the individual puffs can't be heard, but instead what is perceived is a continuous sound. The cutoff frequency below which individual pulses of sound energy can be heard is around 70 puffs of air per second (or 70 Hz), but can vary between about 60 Hz and 80 Hz depending on the listener. If the pitch is below 70 Hz, the perception is of vocal fry. Above the vocal fry in pitch comes the chest register, or chest voice.
Unlike vocal fry, perception of the chest, head, and whistle registers doesn't depend on a cutoff frequency, or on fundamental limits in our ability to hear rapid changes in sound waveforms. Instead, these higher registers are largely perceived based on variations in vocal quality, or timbre.