Proofing active yeast is a simple process of adding dry yeast to warm water, 110-120F, with a tablespoon of sugar. Active yeast will start to foam and bubble also known as blooming. This step is done to prove that your yeast is active for your recipe and will do its thing. Yeast that is no longer active will not have any activity ruining your baked good. Yeast is a live organism, but over time it will lose its activity and if you are looking for a light and fluffy baked good, this step helps you know you will have the desired result.