This tool is used to make a map higher or lower, to change contrast or to correct map gamma.
All of the levels sliders are draggable. Output high and low levels sliders are dragged vertically, while Input low and high and Gamma are dragged horizontally. To reset the slider to it's original position drag out of the node and release it. Reseting Gamma this way will always place it between Input low and high (value 0.5).
In most cases Levels generator can replace the Curve node. While curves are more flexible and give more control, levels node is a bit faster, and has more compact interface. But the main advantage of Levels node over curves is that it's properties could be exposed for function. Curve node key values could not be exposed, so could not be adjusted from the parent graph.
To expose the slider properties, or to enter their values manually unroll the Parameters rollout. Here the values could be edited the way it's done in other generators.