The short answer
A ferran is the neighbourhood's public oven. Most old medina houses have no oven of their own, so each morning trays of shaped dough are carried there under a cloth. The baker slides them onto hot bricks with a long wooden paddle and hands them back later. In some quarters the same fire heats the hammam next door.
In more detail
Walk the residential lanes in the morning and you will see children and women carrying trays of round khobz, the daily bread, covered with a cloth. They are going to the ferran. The baker knows each family's loaves by their marks and their cloth, and the bread comes home hot for the day's meals.
A medina quarter traditionally has its own set of shared services: a mosque, a Koranic school, a hammam, a fountain and a ferran. The oven and the bathhouse are often neighbours, and the fire under the hammam is also where tanjia, Marrakech's clay-urn dish, is buried to cook in the ashes.
Ovens like these are fewer than they were, as more homes bake their own, but many quarters still have one. Bread from them is the khobz you meet at every meal, used as a spoon for tagine.
Our evening food tour stops at a working ferran early in the walk, and La Maison Arabe's cooking workshop also includes a visit to a communal bread oven.