Royalty-free means that you pay a one-time fee for the (usually) unlimited rights to use an image for commercial and non-commercial purposes. Most of the time, you'll need to provide attribution of the creator and database the image was sourced from. Depending on the service, royalty-free images can have certain terms of usage such as the number of print runs. For this reason, it's important to pay close attention to each image's licensing conditions.