In an electrophilic substitution reaction, an electron deficient species attacks the benzene ring which is e‌– rich. 1When an –OH group is attached to the benzene ring, by the virtue of its electron releasing nature increases the e‌– density ofthe ring and thus activates it, i.e., makes it a welcome site for electrophiles.
The increase in electron density can be visualised as :From structures (I) - (V), we find that the attachment of hydroxyl group to benzene has increased the electron density (–ve charge) on the ring carbon atoms (especially C-2, C-4 and C-6). It is therefore said to have activated the ring towards electrophiles which are attracted to the increased electron density.