The idea of proofing yeast originally came about as a way of "proving" that the yeast was still active. This was back in the days when yeast had a lot shorter shelf-life and bakers couldn't always be sure their yeast was still good. Nowadays, yeast can be refrigerated or frozen for about a year and still work just fine. But unless you're baking every day, we still think proofing is a good idea - better safe than sorry!
All you do is measure out the yeast and mix it with the water called for in the recipe. Yeast is happiest at about 75°-80°, so the water should feel barely warm or lukewarm to the touch. Add just a pinch of sugar to give the yeast something to munch on.
Let the yeast and water sit for a few minutes. First, the water will dissolve the dry coating around the granules of yeast, releasing the active yeast inside. The active yeast will go to work on the sugar and a bubbly foam will start to form on the surface from the carbon dioxide being released. This foam is proof that the yeast is active, and once you see it, you can add the yeast to your bread dough.
Do you think proofing yeast is important, or are we just being superstitious?
Related: Food Science: What is Sourdough?
(Image: Emma Christensen for the Kitchn)