You can search for videos on Coverr by entering simple keywords like "dog" and "cat." The other option for discovering videos on Coverr is to simply browse through the thematic collections of stock video clips.
Coverr's user interface does have one element that might cause some confusion for students. Coverr appears to generate revenue through Shutter Stock's affiliate program. That's why there is a row of Shutter Stock video clips on the top of every page and another a little lower on each page. It would be fairly easy for someone to overlook the Shutter Stock label and end up clicking on one of the videos that isn't available for free.
Applications for Education
Pixabay has been my go-to resource for free B-roll video clips for years. Coverr probably isn't going to supplant Pixabay as a my go-to, but it does represent an alternative source for free video clips that teachers and students can use without worrying about copyright infringement.