Casein is the dominant protein in cow's milk, famously slow to digest. In the stomach it forms a gel that releases amino acids gradually over several hours. This makes it useful for supplying muscles during long gaps between meals or overnight.
Most people take casein as a shake mixed with water or milk. The typical time to use it is right before bed, giving the body a steady nutrient supply through the night. It also works between meals or after a workout to extend recovery and fullness. Micellar casein is widely chosen because it digests even slower than calcium caseinate.
Casein is a complete protein and naturally carries calcium. Watch out if you have trouble with dairy: it contains lactose and milk proteins. People with lactose intolerance sometimes get bloating or gas, and anyone with a milk allergy needs to avoid it entirely. Its slow gastric emptying can upset sensitive stomachs.