Adverbs that change or qualify the meaning of a sentence by telling us how often or how frequently something happens are defined as adverbs of frequency.
An adverb of frequency is exactly what it sounds like - an adverb of time. Adverbs of frequency always describe how often something occurs, either in definite or indefinite terms. An adverb that describes definite frequency is one such as weekly, daily, or yearly. An adverb describing indefinite frequency doesn't specify an exact time frame; examples are sometimes, often, and rarely.
These simple rules for adverbs of frequency will help you to use them correctly:
Each sentence contains an example of an adverb of frequency; the examples are italicized for easy identification.
This list of adverbs of frequency contains many of the most common; keep in mind though that there are many other words which can serve in this capacity.