It’s a thick, nasty looking, sticky concoction that appears to be a cross between axle grease and earwax. Historical arms collectors have to clean it off of and out of the bore of their weapons. The military slathers it all over their vehicles before they ship them to servicemen and women around the world. And, yes, even woodworkers have to clear the gunk off their power tools before they can put them to use.
It’s known as Cosmoline, and while cleaning it out of your power tools can be a sticky, gummy mess, it certainly does a great job preventing rust from forming on bare metal parts. This entry at Wikipedia notes the substance’s history, and even goes into some detail about how to clean it off treated surfaces. Something you may not do every day, but it’s worth filing away for when your dream tool arrives at your shop!