The relative acidity is determined by the stability of the conjugate base formed after losing a proton.

• Phenol ($C_6{H}_{5}OH$) forms the Phenoxide ion ($C_6{H}_5{O}^{-}$). The negative charge on oxygen is delocalized over the ortho and para positions of the benzene ring through resonance.
• Ethanol ($C_2{H}_{5}OH$) forms the Ethoxide ion ($C_2{H}_5{O}^{-}$). There is no resonance to stabilize the negative charge; in fact, the $+I$ effect of the ethyl group destabilizes it further.
• Consequently, phenol (pKa $\approx 10$) is significantly more acidic than ethanol (pKa $\approx 16$).