Rubber diaphragm (gasket) used on WaterSoft Provectr venturi / nozzle air-injector assemblies.  Sold Individually. Typically two diaphragms are used per venturi assembly.

Removing a diaphragm will allow more water to pass around the nozzle, resulting in less aeration; adding a diaphragm will direct more water into the nozzle and increase the pressure differential, which will increase the amount of aeration in the water.