The short answer
It is a bowl of small snails simmered in a dark broth of aniseed, thyme, liquorice root and other spices, sold from carts on the east side of Jemaa el-Fnaa in the evening. You pick the snails out with a toothpick, then drink the broth. Many Marrakchis swear by it for a cold or a heavy stomach.
In more detail
Locals call it babbouche or ghlal. The snails are small, and the point is as much the broth as the meat. It is dark, herbal and a little medicinal, closer to a hot tonic than a soup. The carts sit on the east side of the square, each with a big steaming pot and a ring of stools.
Ordering is simple. Sit, ask the price, and the cook ladles a bowl and hands you a toothpick. Twist each snail out of its shell and eat it, then lift the bowl and drink the broth. Leaving the broth is leaving the point.
If you are nervous about street food, this is a sensible first adventurous dish: it is served very hot from a pot that keeps cooking all evening, which is what you want from a stall.
Snail soup is an evening and late-night food, sold from when the stalls go up in the late afternoon until about midnight. Our evening food tour finishes among the carts of Jemaa el-Fnaa, so your guide can show you how locals eat it.