Kant's main work, The Critique of Pure Reason (Kritik der reinen Vernunft 1781A, 1787B), is famous for its investigation into whether there are "synthetic judgements a priori". These are propositions that are not merely analytic, but have a content and are prior to any experience of the world. A posteriori propositions, in sharp contrast, are only possible on the basis of experience of the world; they are empirical. Apart from mathematical propositions, which are analytic because they can be deduced from axioms defining a mathematical entity of some kind, all the science we are fed with today through the media has to be 'evidence-based', that is, based on empirical facts, empirical data gathered from the world on the basis of scientists' experience of the world, whether it be in an experiment or an empirical survey (e.g. mining mountains of data).
That is to say that today's science is based overwhelmingly on a posteriori, factual experience of the world, and (purportedly) not on any a priori knowledge of the world prior to any experience of it. Any theory of any aspect of the world is assumed to be a kind of mental model of the world that first has to be tested empirically through experiment. As such, scientific theories of any kind (purportedly) are not and cannot be a priori. Even those mathematized theories thought up a priori to capture movement and change in the world, such as the weird and wonderful theories of quantum physics or Einstein's theory of general relativity, have to be tested a posteriori by suitable experiments. Or so it is claimed. (Hence, for instance, there is the simplistic verificationist epistemology of a Karl Popper.) Any reputable scientist has to believe in this dogma of experimental verifiability and deny that scientists can only proceed on the basis of a priori presuppositions prior to any conceivable experiment.
In truth, when today's science — as well as all the rest of us following obediently — denies the a priori and fixates on the a posteriori experiential givenness of the world for its evidence, it is coming too late, after the event.
Nonetheless, Kant claims against all empiricism that there are synthetic propositions about the world that can and must be made prior to any experience, even through they are not articulated. This ineluctable synthetic a priori that does not have to be given by experience is formulated in his famous assertion at the climax of the KdrV that the preconditions of possibility of experience of the world are identical with the preconditions of possibility of objective experience of the world. In German: "die Bedingungen der Möglichkeit der Erfahrung überhaupt sind zugleich Bedingungen der Möglichkeit der Gegenstände der Erfahrung, und haben darum objektive Gültigkeit in einem synthetischen Urteile a priori."(KdrV A158, B197) Against all empiricist theories of knowledge of the world, Kant is here claiming, after a long and involved discourse, that the world can only be experienced objectively and that the objectivity of the objects inhabiting the experiential world is constituted within the subject prior to any experience of the world.
The objectivity of objects is an historically specific ontology — first articulated philosophically by Descartes — of how beings present themselves to our understanding, our reason, in their beingness as beings, namely, in their objectivity as objects. On the other, 'inner' side there is the transcendental (i.e. a priori) subject in its subjectivity, whose "Gemüt" (psyche, consciousness) is filled with Vorstellungen (representations) and Anschauungen (intuitions). Among the intuitions there are the pure, i.e. a priori, intuitions of space and time themselves, within which the representations given by the senses are ordered according to the rules of pure (a priori) understanding (Verstand) in such a way that these representations are rendered as objects to our understanding.
Oddly enough, these objects constituted by pure understanding basically conform to Newton's laws of motion and, in particular, to the law of efficient causality! Hence, according to Kant, we live in a world governed by efficient causality among objects that move only in succession along the one-dimensional time of linear time. It should be noted that Kant's concept of the pure intuition of time is that merely of succession in movement. Hence time itself is conceived as derivative of movement rather than conversely.
The subject-object ontology presented by Kant that lays down that the world can only be experienced in its objectivity by a subject endowed with inner consciousness, however, is not the ultimate ontology with which we are stuck forever. Kant's KdrV, in fact, can be — and has been — deconstructed to open the way for an alternative ontology of the being of beings in which there is no longer any subject-object split.
But that is a story for another day. It goes without saying that today's degenerate mainstream philosophy has sold out its raison d'être of questioning deeply and denies that there is any ontological issue at all in Kant's KdrV. Rather, it reads KdrV merely as epistemology, thus deforming Kant's philosophy, and skirting and suppressing the ontological issue entirely through which there could be a radical change in our thinking.