This is a constant characterizing the electron's magnetic moment due to the electron's inherent spin.

Notes

A rotating electrically charged body creates a magnetic dipole and this is seen analogously in an electron.  Any external magnetic field will exert a torque on the electron's magnetic moment and that torque will be dependent on the electron's spin orientation relative to the external magnetic field.

This constant is specified with a standard uncertainty (standard deviation) of: 0.000021 E-26 J/T