The above diagram could represent a typical oscilloscope screen with a 50Hz 230Volt sinewave supply (such as the regular mains) superimposed over the squared output waveform of an inverter. The sharp rising and falling edges of the inverter waveform can lead to inefficiencies in the power output stage when driving capacitive loads (such as I.T. equipment and fluorescent light fittings with large filtering components present on the mains input stage). The sharper (more vertical) the voltage change is the higher the current that will flow in the output stages and the load. In the squared waveform these will be seen as a series of spikes in the output stage current flow, it is this which heats up the switching components whereas the sinewave has a more gradual change which will be reflected in the current flowing through the circuit.
The animation below shows the waveform over time, the current for each type being represented at the point they pass the centre of the diagram. The sharp rising edges of the squared waveform cause a series of high current spikes, typically in a 50Hz output there would be some 200 of these spikes passing through the output circuit every second. The sinewave by comparison has a much smoother change without the high spikes.