The green line in the graph shows, in theory, how a bubble of air at ground level will cool as it rises and expands. When the atmosphere is at low pressure, the rising air bubble is always warmer than the surrounding air. On the graph, the green line is always above the red line. Under conditions of low pressure, the heated air will continue to rise. As it is always warmer than the surrounding air, it continues rising.
Low pressure leads to unstable air conditions.
When the ground is heated, the air will rise and keep rising. When the atmosphere is at high pressure, the rising air bubble is always cooler than the surrounding air. On the graph, the green line is always below the blue line. Under conditions of high pressure, the heated air will not continue to rise. Being cooler than the surrounding air, it will not rise.