The Fast food industry supplies food that is quick to prepare and eat. Some fast food is healthy but typically fast food like burgers is made to taste good with little concern about the effect on the health of the people who eat it. Fast food frequently contains too much unhealthy saturated fat, salt, sugar and other additives. Too much fast food causes obesity.
Fast food may be bought in shops and supermarkets or sold ready cooked. Subway has a reputation for being less unhealthy than other fast food outlets, we're not sure how far that's true.  It's best to limit how much you eat of all fast foods.