The term 'a priori', when used in reference to knowledge, is, primarily, the idea that awareness, (or consciousness, or sentience), as such, includes a knowledge of things which transcend contingent experience.
A secondary usage of the term 'a priori knowledge' is the idea that awareness, as such, includes a knowledge of some contingencies of denying that which is known a priori. For example, if '2 plus 2 equal 4' either is, or is modeled after, a type of a priori knowledge, then this knowledge would include the knowledge that '2 plus 2' does not equal '5'.
A priori knowledge thus is contrasted with the 'subjective experiences' which obtain as contingencies of merely physically sensible things. For three examples, no one knows, by virtue of being aware in general, that there are two plush dog toys on my desk, that there are stars in the space beyond Earth, and that the air at 40,000ft above Earth is so sparse that you would soon die of oxygen depletion were you to try to simply breath it.