It's how the fuel flows through the jets. A DGV has an idle circuit for the primary, and low and high speed circuits for both primary and secondary barrels. The idle circuit is controlled by the primary low speed jet and the idle mixture screw. As soon as the throttle opens off idle, the idle circuit and mixture screw stops doing anything and you're on the low speed circuit, low speed jet only. As RPM and airflow increase, it transitions to the high speed circuit and the high speed jet. Repeat for the secondary barrel.

There's more to it than that, but those are the basics. Weber side-drafts work similarly.