Conceptually, a loop is a collection of cels which, when played consecutively, produce a smooth animation. Often, but not always, the animation can start again at the beginning once it has reached the end, providing a never-ending smooth animation. This is the source of the name "loop."
Technically, a loop is nothing more than a collection of cels in a view resource. Not all loops actually contain looping animations. Some loops are intended to run through a single time and then stop. They can even contain collections of cels that are not intended to animate at all but are instead intended to be used individually. From this viewpoint, loops are an organizational tool for grouping cels within a view.