The site is trying to match the user’s query to the contents of the images (not the image captions or any other image metadata) by using a neural net (source).
These images are not necessarily legally freely usable. If you want legally freely usable images, see the end of this post for 2 different web apps that search site Unsplash.
19M images from Reddit, Instagram, and Pinterest
it doesn’t use CLIP directly, but it does use similar methods
I am not affiliated with this site or anyone involved with it.
Example: search query “a tennis ball in a dog’s mouth”. One of the search results: