Royalty FREE means you do not have to pay fees to the owner for every time you use their product (images, sounds, songs, logos, written materials etc)
If you are a business you need to pay particular attention when using anything that has not been created by you. The penalties can be very high if you are caught.
Here at Learning English in Ireland we have many products that are Royalty FREE and these are clearly marked so that you do not have to worry about using them as often as you like
Royalty FREE does not mean Copyright FREE. The owner of the product may offer it as Royalty Free meaning they are not charging fees but they still own it so you cannot sell it or put your own name on it or you could be prosecuted.