A priori knowledge is that which is derived from reason rather than experience. A good example would be mathematical knowledge. I know that 2+2=4, not through experience, but simply through the operation of my reasoning powers.
One of the most notable features of Kant's philosophy is his application of a priori reasoning to morality. Empiricist philosophers such as Hume argued that morality derives from experience, from longstanding customs and traditions. Reason could certainly help us to clarify our moral goals and how we attain them, but it could not in itself generate motives for moral actions.
Kant would disagree. He argues that moral concepts are indeed a priori in that any moral knowledge worthy of the name can only be the result of careful reasoning. Furthermore, he argues that moral concepts are universal and necessary, in much the same way as mathematical propositions. Returning to our original example, 2+2=4 is a necessary proposition in that the sum, 4, necessarily follows from 2+2. It is also universal in that 2+2 will always make 4, irrespective of the contingencies of time or place.
Kant's fundamental moral principle, the so-called categorical imperative, works in much the same way. This principle states that one only ought to act in accordance with a moral axiom—say, for example, that stealing is wrong—if at the same time one can will it to become a universal law, binding on everyone at all times, all places, and in all situations. As all other moral concepts are ultimately derived from this rational principle, it follows that they too are a priori.