You can change the shape of any curve by moving its curve points. This tool allows you to do that.
Once the tool is activated, all of the green curve points become selectable. If you put your cursor over one, it will highlight yellow.
Click the curve point to select it and drag it anywhere you wish. You'll see the curve shape change as you do so.
You can continue to select and move as many curve points as you want before closing the tool.