Bran is the crushed outer skin of wheat or a similar grain, taken off when people make white flour. The inside of the grain becomes the fine white powder; the bran is the brown, flaky leftover. It is rough and full of fibre, so it shows up in brown bread, bran flakes, muffins, and health talk about digestion. Oat bran is the same idea, from oats instead of wheat. White flour has had the bran (and often the germ) removed; wholemeal keeps them. You buy bran as a dry foodstuff, not as a plant you grow. It is an uncountable noun: some bran, a handful of bran, not three brans.