It is the treatment of another person as if they are vastly inferior in some (or all) ways. Also called patronizing: "Don't patronize me!" (meaning, don't treat me as if I'm an ignorant child or idiot, because I'm not!).
Elitist liberals commonly use condescension to belittle their opponents. Typically this involves belittling the views of conservatives and those who hold tradition or family values, and belittle the lives of rural citizens. Liberals do this to avoid addressing their opponents' views, for which they have no logical response.