Kantians make two distinctions concerning propositions. They are either analytic or synthetic and they are either apriori or aposteriori. The truth of analytic propositions can be established by formal logic alone. Establishing the truth of synthetic propositions requires more than formal logic alone. Observations are necessary to establish the truth of aposteriori propositions and they are not necessary to establish the truth of apriori propositions.
Synthetic apriori propositions, then, refer to propositions whose truth can be established but formal logic is insufficient to do so and observations are unnecessary. What must be added to formal logic is knowledge gained, not by observation, but by reflection.
Take a look at the piece by Hans Hoppe: