The main discovery of Kant is that we are not “blank sheets”, but are born predisposed to certain modes of perception. This thought had profound influence on almost every intellectual discipline.
I believe that Kant discovered that when thinking about the laws of Euclidean geometry (“Given two points, there is a straight line that joins them”). These are “facts” that are deeply embedded in our perception of the world, so much that we cannot imagine a world where they are not true, but at the same time they cannot be proved, they don’t follow from anything.
Therefore, he reasoned, there are laws/axioms/postulates that are “embedded” in our minds, that come before perception giving general direction about our perception of the world. They are “pure reason”.