Stock photos can be used on business websites if the business purchases the rights to the photos, or if the photos are in the public domain. There are many photography companies that have large inventories of stock photos and allow businesses to buy the photos to use however they want.
Stock photos are generic photos of actors doing various nonspecific actions. Many photos include things such as businesspeople sitting around tables, people smiling and talking, people reading or writing, or other random activities. LArge companies take these posed images and sell them to companies who do not want to take photos themselves.
There are various ways for a company to get stock photos onto their websites. The most direct way is to buy licenses to already-taken stock photos. Another way is for companies to find stock photos that are public domain and work for the company's purposes. Finally, companies can either hire a professional photographer or take stock photos themselves. The benefit of buying stock photos is that they are usually professional and pleasing to the eye, they can be bought and put on a website almost instantaneously, and that they are relatively cheap and will most likely end up being cheaper than hiring a professional photographer.