To pasteurize food is to sterilize it, or heat it until harmful bacteria is killed, making it safe to eat. Most dairies pasteurize their milk before bottling it.
When food producers pasteurize their products, they are making it safer for large numbers of people to eat or drink them. When they pasteurize apple cider, for example, it's heated to a certain temperature for a specific length of time and then cooled before being put in containers for sale. The word pasteurize comes from the name of the French chemist who invented this process, Louis Pasteur.