Yeast is a tiny plant-like microorganism that exists all around us - in soil, on plants and even in the air. It has existed for so long, it is referred to as ‘the oldest plant cultivated by man’. Yeast is used to leaven breads.
Active yeast are tiny dehydrated yeast granules of living yeast. Once the granules are hydrated with liquid the yeast granules come out of dormancy and are "active" again.