It refers to the purity and grade of the metal, as well as the level of tempering the material has been through.
http://en.wikipedia.org/wiki/6061_aluminium_alloy
Most manufacturers and metal smiths do not work with 7075 for airsoft slides, because the stock is simply more expensive and it is unnecessary to build it with that much strength. Otherwise, they could build it from even stronger alloys.
7075 is slightly harder to work with than 6061 (shortened tool life creates higher overhead costs), and quite honestly... you don't need it to be that strong for airsoft use.
With the new ILLusion Kinetics outer barrels, impact fatigue against the slide is now negated to ZERO, so that type of strength and added cost is simply unnecessary overkill.
Yes, the product will be stronger, but for what? If the product experiences minimal wear and fatigue to begin with, what's the point of dumping money in to it to produce it from 7075? Or Titanium? Or some other mega super uber unobtanium alloy?