Basically, it illustrates the difference in operation between buck, buck-boost and boost modes. When V_IN is greater than V_OUT, the devices operate in buck mode. When V_IN is less than V_OUT, the devices operate in boost mode. When V_IN is near V_OUT, the devices operate in buck-boost mode. Shown here in this graph, as V_IN decreases, the device moves from pure buck mode into pure boost mode.