The central philosopher of the European Enlightenment and — by common consent — one of the two or three most important philosophers in the Western tradition. Kant's Critique of Pure Reason (1781) tried to settle the quarrel between rationalism and empiricism by proposing that the mind actively structures experience through innate categories (space, time, causation). We can know the phenomenal world (things as they appear to us) but not the noumenal world (things in themselves). This is the "Copernican revolution" in philosophy.
Ethically, Kant grounded morality in reason alone. The categorical imperative instructs us to act only on maxims we could will as universal laws, and to treat persons always as ends in themselves, never merely as means. This is deontology's foundation and the sharpest rival to Utilitarianism: for Kant, you cannot torture one to save five because doing so uses a person as a mere means, full stop.
Politically, Kant was a liberal and a republican who celebrated the The French Revolution (while deploring the Terror). His late Perpetual Peace (1795) sketched the conditions for a peaceful international order — republican constitutions, a federation of free states, universal hospitality — that foreshadows modern liberal internationalism. John Rawls explicitly took himself to be working in the Kantian tradition.