Optimum basically means best. So the optimum level for any organism is the condition where that organism thrives. The optimum temperature for a palm tree is not the same as a redwood tree. At optimum conditions, the organism is able to live and reproduce easily and you would expect to find the majority of organisms in these conditions. If you increase or decrease that particular condition (like temperature) then the organism is living under a little bit of stress and it will not be as "happy" or live quite as comfortably. If the condition is too far away from the optimum condition, the organism will not survive. Sometimes we draw a bell-shaped graph to illustrate where the optimum conditions are and we call the graph a range of tolerance.