The statement that the mass of the electron is a relative mass means that its value is expressed in comparison to a standard mass unit or reference. In scientific contexts, the relative mass is often used to describe the mass of a particle relative to the mass of another particle or reference standard.
The most common reference for expressing the relative mass of the electron is the atomic mass unit (u) or the unified atomic mass unit (u), which is defined as 1/12th the mass of a carbon-12 atom. The relative mass of an electron is approximately 1/1836 atomic mass units (or 1/1836 u).
This value indicates that the electron is significantly lighter than most other subatomic particles. For instance, compared to the proton, which has a relative mass of approximately 1 atomic mass unit, the electron's mass is about 1/1836th that of a proton.
It is worth noting that the term "relative mass" is sometimes used interchangeably with "relative atomic mass" or "relative molecular mass" when discussing the mass of atoms or molecules. In those cases, the reference is the mass of an atom of carbon-12, which is assigned a relative mass of exactly 12 atomic mass units.