The output voltage cannot exceed the range determined by the power supply. So if the input voltages are such that the output tries to exceed these limits, we get saturation. This is illustrated in Figure 8. You can see that the waveform gets clipped if it tries to exceed the power supply limits.
In this course we will assume that the voltages are always within the unsaturated range, and so we ignore saturation effects, unless otherwise stated.
ANU Engineering - ENGN2211