Condescension is an insulting way of talking to other people, as if they were stupid or ignorant. Condescension is rude and patronizing.
Treating someone with condescension is the opposite of treating them with respect. Condescension is full of arrogant and snooty attitude, and people who practice condescension treat others like inferior idiots. Sarcasm goes well with condescension if you're trying to be a jerk. You can almost hear the word descent in condescension, so think of someone acting all high and mighty, looking down at everyone else.