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In organic II, you learn about aromatic rings and how stable they are thus unreactive. But when you add a substituent to the ring, it can become more reactive or even less reactive. When adding a substituent makes the ring more reactive, it is called activating, when the substituent added makes it less reactive, it is called deactivating. Usually groups that have a lone pair on the atom connecting to the ring directly are activating and thus makes the ring reactive. It is able to donate that lone pair into the ring to attack another "substrate" for a reaction. If the atom attached to the ring is attached to an electronegative atom also, then there is a good chance that the substituent as a whole will be deactivating. (electrons are getting pulled away from the ring, thus unreactive)
Two factors are in play, inductive effect vs. resonance effect when determining if a group is activating or deactivating. Then after you get this down, look into ortho/para directing vs. meta directing. Hope this helps.