This video explains what a royalty-free stock image is. Here is also a written explanation.
When you purchase a royalty-free license to an image, video or other media product, you pay a one-time fee to obtain the non-exclusive lifelong right to use the product.
You can choose how to use the media product, how many times you use it, how long you use it for, how many projects you use it for, etc. You can use the media product legally without additional fees or interruption from the copyright owner, as long as the use of the product conforms with the license agreement.
There will always be restrictions however. Normal restrictions will be that the content cannot be re-sold or transferred to another owner, and that you cannot use the products in an offensive, discriminatory, etc. fashion. For certain royalty-free products, you may also find commercial restrictions, for instance specifying upper limits on number of copies you can produce. It is recommended to always read the license agreement to understand the full scope of use.
The key feature of a royalty-free license is that it can be obtained by several users. Meaning, several people can purchase the right to use the product at the same time. what that means, is that none of the users of a royalty-free license has the exclusive right to the licensed product.
This also means that even though you have obtained the right to use the royalty-free product, you do not own the copyright of the product or the product itself. There will be other people just like you, who also have the same rights to use it.