Can someone explain part of this wiring diagram to me?
In normal North American power wiring, we get two 120 volt "hot" wires that are 180 degrees out of phase, so if we measure between those two hot wires we will get 240 volts. (Electrical Code rules call these 125 and 250 volt.)

Where there is three-phase service (many commercial and industrial users), normal outlets still get a single phase 120 volt supply.  But if you connect between two phases (two separate "hot" wires), you will get 208 volts, as the phases are 120 degrees apart, rather than 180.