The short answer
Usually a fixed menu of several courses in a restored courtyard house: cooked salads, pastilla, a tagine or couscous, pastries and mint tea, often with live music. Dar Moha, Ksar Essaoussan, Dar Essalam, La Maison Arabe and Riad BE are well-known examples. It is a slow, evening meal rather than a quick dinner.
In more detail
The setting is half the meal. Dar Moha, in the former home of couturier Pierre Balmain near Mouassine, serves fixed-menu Moroccan feasts around a banana-tree courtyard pool, with Gnaoua musicians playing between courses. Ksar Essaoussan, a 16th-century riad near Jemaa el-Fnaa, has a doorman in red and a seven-course tasting menu of bstilla, seasonal tagines and couscous on a terrace over the medina.
Dar Essalam, founded in 1952 inside a restored 17th-century palace, is one of the medina's oldest continuously running dining houses. At La Maison Arabe, founded in 1946 by the Bab Doukkala mosque, Andalusian musicians play beside an open fire while chefs plate lamb tanjia and pastilla. Riad BE serves a fixed Marrakchi Surprise dinner of home-style tagines beside a zellige plunge pool under the open sky.
Expect it to take the whole evening. Courses come one after another, and the music sets the pace. Most of these places sit down lanes where cars cannot reach, so ask the restaurant how to find the door from the nearest gate.
Alcohol is served at some and not others, so ask ahead if it matters. Tell the restaurant about vegetarian or other needs when you book, since menus are fixed.