it's g-2, rather than -2g, and ...
Muons have a thing called "spin", and "spin" interacts with a magnetic field to make the particle spin around. The strength of that spinning depends on precisely the particle's charge, divided by twice its mass, times some constant. About 100 years ago, when this was first studied, scientists made the prediction that this constant, called "g", should be exactly 2. Later, new calculations showed that it was subtly different from 2. Hence the "g-2", which is to say, "g minus 2", which should be very close to, but not exactly, zero. The remarkable thing is that we think we can make a very precise prediction of this value, and we can also measure it just as precisely, down to just a few hundred parts per billion!
The theory prediction is that g-2 = 0.002331836, and the experiment today gives g-2 = 0.002331841, so the difference is tiny, in the eighth decimal place, but the precision of both is so great that we can make the claim that they really are different and it isn't a calculator error (although stay tuned for updates to this over the next couple of years!)