Most SSPs let publishers set price floors as a way to maintain control over the pricing of their inventory. The primary purpose of a price floor is to let publishers not allow the sale of an ad under a specific price. This type of price floor is referred to as a hard price floor (HPF).
For example, if you set an ad’s HPF at $4.50, bidders offering less than $4.50 CPM (e.g., $4.00 and $4.19) will be automatically rejected.