Intertrial coherence (ITC) is a measure of how consistent oscillatory phase is across an ensemble of trials.
In the schematic below, we have a sine wave of an arbitrary frequency.
The amplitude of the wave is the distance between 0 and the wave at a given timepoint. So here, you can see it has a maximum of 1, indicated by the arrow. A complete cycle of of the wave is the amplitude peaking, declining to a trough, then hitting zero again.