Foods from all groups can belong to this broad category.
Those foods are generally unhealthy for the following reasons:
They contain additives, which can be unhealthy, toxic or dangerous (though generally additives in foods are kept within safe limits)
They can contain trace amounts of plastic, that leaks from packaging, and which can cause hormonal disruption.
They usually have too much unhealthy fats, salt and sugar.
They can also contain extremely unhealthy trans fats.
They are usually low in beneficial nutrients, and provide a lot of “empty calories”
They are produced with the philosophy which cares predominately just about taste, they taste good but provide little nutrients, and can be psychologically addictive.
They are very fast and convenient which removes you from all the parts of the process except consumption, they are instantaneous which promotes laziness and obesity.
However, not all foods from this group are the same. Try to eat such foods as rarely as possible, and when you do eat them, read the labels.
For example high quality Italian pizza or ice-cream is definitely better than potato chips, even though both belong to this questionable category.
Fast food is generally what comes from chains of fast food restaurants, such as McDonalds. The same foods, for example pizza and barbecue, might not belong to the category of fast food if they are from another, high quality restaurant or Italian pizzeria, and not a fast food store. In restaurants usually better ingredients are used, and such foods can have better quality.