Purchase Frequency is a panel data measure. It is the number of times your average buying household purchases your product over a whole time period (usually a year). Purchase Frequency remains the same regardless of which sales measure is used (dollars, units or EQ volume). In Nielsen the fact is called Item Trips per Item Buyer. In IRI the fact is Product Trips per Buyer.
If annual purchase frequency for Brand X is 4.2, it means that each household who bought Brand X, on average, bought it 4.2 times over the course of the year.
Sales = (Total # of Households x Penetration) x Buying Rate
Buying Rate = Purchase Frequency x Purchase Size
Reference articles: Panel Data Dictionary, The Chart Every CPG Analyst Should Understand.