This constant represents the ratio of the mass of a muon to the mass of a tau particle.
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
The tau (`tau`) is also called the tauon or tau lepton and has a negative charge and a spin of 1/2.
This constant is specified with a standard uncertainty (standard deviation) of: 0.000 54 E-2