This constant represents the ratio of the magnetic moment of a muon to the magnetic moment of a proton.

A muon is an elementary particle classified as a lepton which has properties similar to an electron: it has spin of -1/2 and charge of -1.  The muon however has significantly more mass than an electron.  See Muon Mass

Notes

This constant is specified with a standard uncertainty (standard deviation) of: 0.000 000 084