Flux can be used to refer a number of different things, actually, as you can see here:
1. a flowing or flow.
2. the flowing in of the tide.
3. continuous change, passage, or movement: His political views are in a state of flux.
a. the rate of flow of fluid, particles, or energy.
b. a quantity expressing the strength of a field of force in a given area.
5. Chemistry, Metallurgy.
a. a substance used to refine metals by combining with impurities to form a molten mixture that can be readily removed.
b. a substance used to remove oxides from and prevent further oxidation of fused metal, as in soldering or hot-dip coating.
c. (in the refining of scrap or other metal) a salt or mixture of salts that combines with nonmetallic impurities, causing them to float or coagulate.
–verb (used with object)
7. to melt; make fluid.
8. to fuse by the use of flux.
9. Obsolete. to purge.
–verb (used without object)
10. to flow.
But as you can see, it mostly has to do with things flowing. So mass flux has to do with flowing mass. To be more precise, mass flux is the mass that passes through a given area in a set period of time.
It's like if you had a pipe of water or refrigerant or some other fluidy-type substance. The pipe has a fixed cross-sectional area (the area of a circle). And then you measure how much mass of your fluidy-type substance is flowing through that area for a set length of time, say 1 second. And that would be your mass flux.
Here is a picture to help illustrate:All the red dots are supposedly the mass flowing through the cross-sectional area in one second.
Mass flux is symbolized by "G" in all the papers I have been reading. So of course I had to write about it.