Yeast is a living organism and should be stored properly and used by its expiration date for best results. To ensure it stays fresh until then, store unopened yeast in a cool, dry place like a cupboard. Once it’s been opened it should be stored in an airtight container in the fridge or freezer. If stored in the fridge or freezer, let it come back to room temperature before using it.
More baking tips can be found on Red Star Yeast’s online Lessons in Yeast and Baking.