It is mass-produced food. It is often western pattern and pre-prepared and delivered to the shop ready for frying or boiling. There is a strong priority on "speed of service".
The restaurants that sell fast food are called "fast food shops" or "fast-food restaurants". Some of the more common fast food restaurants are McDonald's, Wendy's, Burger King, Culver's, Taco Bell, and KFC. All these chains have a limited short menu and serve the same kind of food in all their outlets. The shops are subcontracted by the supplying company and tied absolutely by contract to that single supplier. However, the same kind of criticism could be (and is) directed at some other kinds of restaurants. The problem is seen by many critics as being one of over-eating, rather than one of the specific food types.
Fast food is often considered unhealthy due to it being loaded with calories, sodium, and unhealthy fat—often enough in one meal for an entire day. It also tends to be low in nutrients and almost totally lacking in fruit, vegetables, and fiber. Eating too much fast food can cause a human to have poor quality health which is linked to a higher risk of obesity, depression, digestive issues, heart disease, and stroke, type 2 diabetes, cancer, and early death.