Yeast is used,most commonly, in making bread. After mixing the dough, yeast metabolizes sugar molecules, generating carbon dioxide in the process. The resulting gas, when rises, will cause the dough to rise as well and hence it is used as a leavening agent in making breads. It has been found experimentally that at 0 degrees Celsius, no carbon dioxide is formed. From 10 degrees to 40 degrees temperatures, carbon dioxide generation increases and decreases afterwards. Temperature affects all the rates of reactions, as per Arrhenius equation. So, initially the gas production rises, however at higher temperatures, the rapid rate of sugar consumption is not sustainable (since the amount of sugar is fixed) and hence the rate of gas production falls. One can also check cookbooks to find out the most optimal temperature for bread making.
Hope this helps.