When a benzene ring already carries a substituent, that group controls two things about the next substitution — how fast it happens (activation vs. deactivation) and where it happens (which position). This directing logic is one of the most heavily tested topics in all of Orgo 2, and it reduces to a single unifying idea: substituents act by donating or withdrawing electron density, and the effect on rate and on position both follow from that.

Activating, ortho/para-directing groups are electron donors (–OH, –OR, –NH₂, alkyl groups). They push electron density into the ring, making it more reactive than benzene, and they stabilize the arenium-ion intermediate best when the new group enters ortho or para. Strong donors (–NH₂, –OH) are powerfully activating.
Deactivating, meta-directing groups are electron withdrawers (–NO₂, –C=O, –SO₃H, –C≡N, –NR₃⁺). They pull electron density out, making the ring less reactive, and they direct the incoming group meta (because ortho/para attack would place positive charge directly on the carbon bearing the withdrawing group — destabilizing).
The halogens are the important exception: they are deactivating (electron-withdrawing by induction) but ortho/para-directing (their lone pairs donate by resonance). This "deactivating but o/p-directing" combination is a classic exam trap.

The payoff is multi-step synthesis planning: the order in which you install groups determines the substitution pattern of the final product. To make a meta-disubstituted benzene, install the meta-director first; to make an ortho/para product, install the o/p-director first. Reasoning through directing effects to sequence reactions correctly is the capstone skill of aromatic chemistry.

"Point the Way"