It is highly recommended that you see the video on mixed and entire radicals prior to checking out this video.

https://mathpqjq.com/mixed-and-entire-radicals/

Radicals need to be “like” in order to add or subtract. Explore this process in the following video and then be sure to try the practice questions.

If you are interested in some examples with cube roots, then see https://www.youtube.com/watch?v=rvUzb0KLzCA