This constant represents the Compton Wavelength of a muon divided by 2 `pi`. This reduced Compton Wavelength is a common representation of mass for quantum mechanics applications.

The Compton wavelength is a quantum mechanical property of nuclear/atomic particles that is equivalent to the wavelength of a photon having the same wavelength as the rest mass of the particle.  So, in this case, the Compton Wavelength of a muon represents the wavelength of a muon's rest mass.

Notes

This constant is specified with a standard uncertainty (standard deviation) of: 0.000 000 047 E-15 m

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