A flash unit is a heating panel that swings or indexes over the platen for a few seconds. It is not a full cure. The aim is to gel the ink just enough that it feels dry and will not lift onto the next screen; the final cure still happens in the conveyor dryer. Ink makers warn against fully fusing a flashed white, because colors printed over a completely cured layer may not bond well to it.
Flashing is most common after a white underbase on dark garments, before puff or other specialty layers, and between colors that would otherwise blend. Each flash adds seconds to every print and more heat to the garment, which matters on polyester blends that can scorch or release dye.
If a quote mentions flash stations, that explains why the job runs slower than a simple one-color print. Ask how many flashes your design needs; reworking the art so colors print wet-on-wet can sometimes remove one. See how screen printing works.