What is Equivalent protons?

Example 38.4 (Counting groups)

Propanone, $\ce{CH3-CO-CH3}$, has six protons but one group: the two $\ce{CH3}$ are alike, one on each side of the $\ce{C=O}$. Ethanol, $\ce{CH3-CH2-OH}$, has three groups ($\ce{CH3}$, $\ce{CH2}$, $\ce{OH}$), and ethyl ethanoate, $\ce{CH3-CO-O-CH2-CH3}$, three as well: the $\ce{CH3}$ next to $\ce{C=O}$, the $\ce{CH2}$ next to $\ce{O}$, and the $\ce{CH3}$ at the end.