The Sync shape is used to generate the current value based on location to the bar or beat. It is similar to the Pulse shape -- in broad terms, think of Pulse as a processor and Sync as a generator: Sync takes the current bar/beat location and uses that to find a location in its internal shapes to generate a new value.
The Sync shape can have one or two internal shapes. The first is used for the bar offset (and the beat offset, if no beat shape is present but the Beat Amount is not 0), the second shape is used for the beat offset.
Sync is an internal shape used to build other shapes, and is available only in the shape editor. To understand how it works, read the Sync Stepper description.