Frequency refers to how often an event occurs. High frequency events, like waves, are measured in Hertz. Lower frequency events, like comets, are often referred to by terms like "twice a century" or even how many years until an event reoccurs. Events like comets happen with predictable regularity, but a "once in 100-year flood" does not refer to frequency, rather statistical probability (every year there is a 1/100 chance that the flood would occur).
To learn more about frequency from a Physics perspective, check out Hyperphysics.