This constant represents the molar mass of a helion, a nucleus of helium-3.  
In chemistry, the molar mass  M  is a physical property. It is defined as the mass of a given substance (chemical element or chemical compound) divided by its amount of substance. The base SI unit for molar mass is kg/mol.  The molar mass of a substance is the mass of 1 mol of the substance.

Notes

This constant is specified with a standard uncertainty (standard deviation) of: 0.000 000 0025 E-3 kg/mol