A loaf (pl.: loaves) is a (usually) rounded or oblong quantity of food, typically and originally of bread. It is common to bake bread in a rectangular bread pan or loaf pan because some kinds of bread dough tend to collapse and spread out during the cooking process if not constrained; the shape of less viscous doughs can be maintained with a bread pan whose sides are higher than the uncooked dough. More viscous doughs can be hand-molded into the desired loaf shape and cooked on a flat oven tray.
https://en.wikipedia.org/wiki/Loaf