It is basically paint without pigment. It is a little confusing as createx refer to it in in terms of reduction. However what is does is actually reduce the amount of pigment by volume so you can make colours less intense/lighter shades/increase transparency, and this can also help with flow. It should however be used in conjunction with reducer to reduce viscosity for best performance (this is just my take and what I have found works best for me, a Createx Wicked fan using it on many surfaces.)
It is also good if you have over reduced, but need to give more body back to the paint. T-shirt/fabric artists find it useful as a base coat - when heat set it flattens nap and gives a good surface to paint on, and it can also be used as a light intercoat/topcoat although I haven't used it for this reason.
Useful stuff.