It is an agent that changes the viscosity, making it less runny---------used to keep the epoxy from running off or sagging.
It is often used with either flox, or micro.
Flox is used to add bulk or thicken the epoxy to a putty like state, and it greatly enhances the strength of the mixture. Very hard to sand when set up. Consistency of the mixture can be varied as needed to do the job at hand.
Micro is also used to bulk or thicken, but it is not very good in the strength arena----------but it is lightweight and easy to sand. Consistency of the mixture can also be varied as needed to do the job at hand, very often mixed quite "dry" as a filler.