There are two different ways of making phenols more acidic with substituents. Electron withdrawing groups, and mesomeric withdrawing groups.
Electron withdrawing groups, (eg Cl) will pull electron density out of the ring, which will in turn pull electron density away from the oxygen of the OH, making it more acidic.
If you place a substituent on the ortho or para positions that withdraws charge mesomerically (eg NO2), you can draw out canonical structures that show the delocalisation of oxygen's lone pair into the nitro group. This drastically decreases the proton affinity for the phenol. Mesomeric stabilisation only occurs on the ortho and para positions, as conjugation is lost when delocalising charge into a meta substituent. This is easily observed by the construction of the relevant canonicals.