The elementary charge $e$ is a natural unit of electric charge associated with electrons and protons. A proton has charge $+e$ and an electron has charge $-e$.

In SI units,

$$e=1.602176634\times10^{-19},\mathrm C.$$

This value is exact because the modern SI defines the coulomb using a fixed numerical value of $e$.

When an ordinary atom gains or loses electrons, its net charge changes in steps of $e$. An atom that loses two electrons, for example, has net charge

$$+2e,$$

while one that gains three electrons has net charge

$$-3e.$$

This discrete behavior is called charge quantization. Ordinary ions therefore carry net charges that are integer multiples of the elementary charge.

At deeper levels of particle physics, some constituent particles have charges expressed as fractional multiples of $e$. That does not change the role of $e$ as the conventional charge scale for electrons, protons, atoms and ions.