This constant represents the molar mass of a proton The molar mass, M, is defined as the mass of a given substance (chemical element or chemical compound) divided by the amount of the substance. 

Notes

This constant is specified with a standard uncertainty (standard deviation) of: 0.000 000 000 090 E-3 kg/mol

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.