The Stocks is more or less a search engine where web designers can browse and purchase royalty-free pictures, videos, and many more. These utilities are sold by companies advertised on the site itself. Technically The stocks serves as a hub area for other companies to showcase their goods.
As depicted by the name, this website sells stock photos, videos and more. Stock anything, in this case, refers to the professional aspect of distributed media. Many examples of stock photos/videos include horizons, people walking down a busy street, or even animals. These things are not free to use. There are drawbacks when it comes to these; such as the limitation of using a photo/video once per license. Also, most photos, in particular, have obnoxious overlays over them which show the company logo to protect the rights of the property.
Stock videos tend to be a tad bit more expensive than their photo counterparts. These videos are much more sought after which drives that high price. On this site, there are some companies that sell both photos and videos such as one of the more well-known brands; Shutterstock.
I highly recommend using this site if you are A web designer or content creator. The prices are cheap and the platform is simple.👍