backflow protector ?

What are the backflow protectors for? I bought freemie cups and recently bought the medela freestyle pump i used them together once and go milk in my tubing .. now does the protector stopmilk from going into my tubing? I sit upright when using freemie because milk will go into the tubing if slouched or bent over but I didn't do any of that so can someone explain?... I'm trying to increase my supply because I'm boob feeding not really storing.. taking fenugreek and drinking mothers milk I just need help understanding thanks!