Tempo modulation is when a tempo track is modulated by one or more modulator tracks having a modulation type of Tempo. The modulator tracks also must have non-zero Duration, in other words each modulator has its own range of tempo scaling. Initially you can just set them all to 100. Like the tempo track, a tempo modulator can have any function in its steps. The actual tempo at any time is the product of the base tempo, the tempo track, and all of its unmuted tempo modulators. The idea is that tempo can change in polymeter, for example changing slowly in one meter, while changing faster in some other meter.
During playback, the net tempo is shown in the status bar.