Phenol is hydroxybenzene, which has a proton on the hydroxyl (
When you deprotonate phenol, its anionic form has multiple resonance structures:
Because of the electron delocalization, the anion (conjugate base) is stabilized (relative to an aliphatic alcohol), and so the phenol molecule itself is more acidic than an aliphatic alcohol.
(The conjugate acid of a weaker base than an aliphatic alcohol is a stronger acid than an aliphatic alcohol)
For example, the
Or, relative to benzene, the present
That makes the hydroxyl proton more partially positive than in benzene, whose hydrogens are all belonging to the aromatic ring. That means it's more acidic, and so phenol is more prone to deprotonation than benzene.
As a note, the