The frontier shows the best combinations a country can reach. At P it has plenty of economic output and little environmental protection; at T the reverse. Moving along the curve from P toward T buys protection at the cost of output, and economists have little to say about where a society should sit: a low-income country may reasonably weigh output more heavily, a richer one protection.
What economists do agree on is that a point like M, inside the frontier, is a mistake. From M the country could have more output for the same protection, at Q, or more protection for the same output, at S, or some of both. Command-and-control rules sometimes land a country at M; market-oriented tools are a way of getting back to the frontier. The slider moves M in and out, and the side panel says how much output and protection are being given up.