It's essentially probiotic maize porridge turned into a drink — the tangy sourness comes from natural lactic-acid fermentation, not added flavoring.
Mageu is made by cooking maize meal into a thin porridge, adding a little flour as a starter, and letting it ferment for a day or two until it turns pleasantly sour and slightly fizzy. It's sold in cartons at spaza shops and street stalls across South Africa, and is a common homemade staple too.