The short answer
Not much. Marrakech is dry, with rain falling on some days from about October to April and almost none from June to August. Even in the wetter months most days are dry and sunny. Our sources disagree on which month is wettest, so we treat November to March as the season for a few showers.
In more detail
Our month-by-month table describes rain in words, not figures, because the published averages disagree: some put the wettest month in November, others December or March. What they agree on is the pattern. Showers start around October, a few rainy days come through winter and early spring, rain is rare in May and very rare in June, and July and August see almost none.
When it rains, it matters more than you might think. The medina's lanes get muddy and slippery, riad courtyards are open to the sky, and rooftop restaurants move indoors. A light rain jacket and shoes with grip cover it.
Rain in the mountains is different from rain in the city. A wet day in Marrakech can mean snow on the High Atlas, which can close the Tizi n'Tichka pass in midwinter, and the Ourika river runs fast after rain or spring snowmelt. Ask your driver to check before a mountain day.