It allows the CO2 produced by the fermenting yeast inside the bottles containing a water and sugar solution to flow into the grow chamber,
The way you use it is you connect a tube through a hole on the bottle cap, the other end of that tube to one side of the one way valve (in a way that it allows flow from the bottle to the other side), then you connect another tube to the other side of the valve, end the other end of it you put inside the chamber through a hole on the pvc board.
I think it's used because once you close the bottle, the yeast will consume all the oxygen inside for reproduction. Then while it ferments the sugars, it generates CO2. If you didn't use the one way valve, oxygen would keep flowing inside the bottle, and allowing the yeast to perform reproduction instead of focusing only on fermentation.