The acidic nature of phenol is due to the formation of stable phenoxide ion in solution
$C_6H_5 OH + H_2 O\rightleftharpoons \underset{\text{ Phenoxide ion}}{ C_6H_5O^{-} } + H_3O^{+}$
The phenoxide ion is stable due to resonance.
The negative charge is delocalized in the benzene ring which is a stabilizing factor in the phenoxide ion and increase acidity of phenol. whereas no resonance is possible in alkoxide ions $(RO-)$ derived from alcohol. The negative charge is localized on oxygen atom. Thus, alcohols are not acidic.