Fast food is prepared and served quickly and inexpensively. If you're hungry, in a hurry, and have only five dollar in your pocket, you might stop at a fast food restaurant for a burger and a soda.
When it comes to fast food, speed is important, as is obvious from the name, which was coined around 1950. People who eat fast food want something that's tasty, cheap, and fast. Fast food restaurants are known for exactly those things. Along with these mass-produced meals come downsides including low wages for employees and food that's often high in fat, salt, and sugar.