A preconception is an idea or opinion you have about something before you really know much about it. You might have the preconception that all New Yorkers are rude until you visit the city and start meeting friendly people.
Imagine you give your friend an assessment of a popular TV show, saying, "Oh, it's so boring!" despite the fact that you've never seen it. Your negative opinion of the show is a preconception — you may be correct about the show, but you won't know until you actually sit down a watch a few episodes. Preconception comes from pre, "before," and the Latin concipere, "to take in and hold."