What's priori?

A priori knowledge or justification is independent of experience (for example 'All bachelors are unmarried'); a posteriori knowledge or justification is dependent on experience or empirical evidence (for example 'Some bachelors are very happy'). A priori justification makes no reference to experience; the issue concerns how one knows the proposition or claim in question—what justifies or grounds one's belief in it. Galen Strawson wrote that an a priori argument is one of which "you can see that it is true just lying on your couch. You don't have to get up off your couch and go outside and examine the way things are in the physical world. You don't have to do any science."

Taken from Wikipedia.