Yeast is very sensitive to temperature. Cold temperatures slow yeast activity down, warm temperatures speed yeast activity up, while hot temperatures will eventually kill the yeast. Yeast begins to die around 140°F/60°C.
If you are adding the yeast into the dry ingredients of your dough, you want to use liquids that are around 120–130°F/49–54°C. This will feel very warm, but not hot to the touch. If you are adding the yeast directly into the liquid (to proof the yeast), you want to keep the liquid temperature around 110–115°F/43–46°C.