Milk kefir is a fermented dairy drink, known for its probiotic content and potential health benefits. It's made by combining milk with kefir grains, which contain bacteria and yeast. Kefir is Milk kefir is a fermented dairy drink, known for its probiotic content and potential health benefits. It's made by combining milk with kefir grains, which contain bacteria and yeast. Kefir is a good source of nutrients like protein, calcium, and B vitamins, and it may improve digestion, boost the immune system, and support bone health.