Azrou lies at a crossroads in the Middle Atlas Mountains where the Fes to Marrakech (N8) and the Meknes to Midelt (N13) main road cross.  It's always been an important market trading town and on Tuesday mornings there is a large regional souk on the outskirts of town. The Lonely Planet guide has a map that will allow you to easily find the location.  See also

As that guide book states, this is NOT a tourist souk. Rather it serves as a market for the local people in the region. Consequently it has interest beyond the standard fare one finds in Fes or Marrakesh.  For one thing there is a large section devoted to buying and selling sheep and goats, and lots of fruits and vegetables are for sale.   Some of these are 'wholesale' where truckloads of one particular fruit or vegetable are unloaded, other areas are 'retail' where merchants resell the stock they've bought.  

If you arrive before 9am you may find that many vendors were in the process of setting up their stalls. There was already plenty to look at, buy and eat, but the market was not yet in full swing.   You can often find a parking spot not too far from the entrance and to stroll around a relatively uncrowded grounds. If it were raining, or, if it had rained heavily during the day or two before, the grounds might be muddy.

The souk has a gritty quality, and folks do not appreciate having their pictures taken without permission. Wandering around the grounds provides much more of a glimpse into every day life in Morocco than one finds visiting the big cities, so going there was definitely worth the time and effort.