To patronize someone in a condescending way is to treat them as if they're in need of extra help because they're not capable by themselves. This is often done with a patronizing tone. Condescending and patronizing are often used as synonyms, but they can have slightly different shades of meaning.
Patronize comes from Latin patronus "protector, master," related to pater "father." So if you patronize a person, you talk down to them like a father might do to his child or a master to his apprentice.
