Although the name might be leading you to think you can use them without paying anything (the free photo part might be misleading), it’s important to clarify that this is not the case. The “Free” in “Royalty Free” concept refers to the fact that you don’t have to pay royalties to the owner of the image (nor to the agency selling the license) every time you use the photo, nor every time the photo generates your profit.
You do have to pay for the rights to use the royalty free photo in the accepted terms, but only once, and then you can use the photo as many times you want, forever.