I generally hear that when someone feels that they are being talked down to, meaning that someone else is talking to them as if they are a little kid or don't know very much. Example: "Honey, don't forget to dry the dishes and put them away after you wash them." "I know, mom! You don't have to be so patronizing with me." or "Would you mind if I gave you just a little bit of advice that might possibly help you?" "You don't have to be so patronizing. Just tell me!"

It means that some is acting superior to you in a certain way. They are traeting you as if your stupid.