From what I recall -- it has been awhile since I was forced to read the Critique -- Kant taught that human beings, in order to think/learn anything at all about the world (a posteriori), must presuppose certain concepts: namely, space and time. These may be 'true' or may be merely mental constructs; we do not know. He called these concepts "a priori synthetic." Again, a priori synthetic concepts are just the necessary presuppositions behind a posteriori knowledge. We need them to know things.
If I am not mistaken, this framework is at work (sort of, anyway) with the Categorical Imperative, this moral principle being synthetic a priori. In order to find out whether act X is moral or immoral (something we should do or should not do) -- or, more to the Kantian point, whether a moral statement is true or false -- we need the Categorical Imperative.
Kant makes my head hurt...