a Venturi system just means that: one thing is somehow injected into another thing by means of a device that is specially designed to enable the one to mix with the other. In the case of a Jacuzzi, air gets injected into the water line by means of a specialy designed Jacuzzi jet.

On top of your Jacuzzi there should be a dial that you can turn, that is marked Max-Min. All this does is suck air into the water line and in this way boosts the jet pressure. You will know that the Jacuzzi Venturi system has been activated when you see bubbles coming out of the water jets.