The VB pin is the bias source for the upper gate driver. The VS pin is the reference for the upper gate driver. The diode and capacitor that are placed on those pins create a charge pump that allows the upper driver to have a bias that is referenced to the source of the upper FET since the upper FET does not have its source tied to ground. When the lower FET turns on, the VS pin is pulled low and the diode charges up the cap across VB and VS. When the lower FET is turned off, the flying capacitor is able to supply gate voltage to the gate-source of the upper MOSFET.