#2. Proof Your Yeast
When you proof yeast, all you're doing is proving that it is alive. That it is eating sugar and emitting bubbles of carbon dioxide, because that is what yeast does.
If you are starting with brand new yeast well within its use-by date, it is not strictly necessary to proof every time you bake, especially if you make bread frequently. But, if you found some yeast shoved into the back of your cabinet or you haven't baked bread in months, it is best to err on the side of caution and prove to yourself that the yeast is alive.
If you are using a bread recipe that calls for putting all your ingredients, including the yeast, in the mixer together and turning it on, warm up a portion of the water called for in the recipe. Yeast will die in temperatures of over 140°F anyway, which defeats the purpose of proofing your yeast in the first place.
Don't stress over the temperature too much. As long as it feels warm and comfortable to you, it will be warm and comfortable for the yeast. Add a tiny pinch of sugar, squirt of honey or splash of maple syrup, just enough to give the yeast a reason to wake up and eat.
Stir everything together and wait 10-15 minutes. If the mixture is nice and foamy with a dense head on top (kind of like the head on a freshly pulled pint of Guinness stout) you're good to go. If you don't see any bubbles, let alone foam, the yeast is dead and you'll need to buy more.