Divine Revelation is the set of truths which God has revealed to us. God has given us the gift of reason, through which we can gain knowledge of many truths about Him. However, God wanted us to know more about Him than that which we could learn on our own, and thus, He taught us many profound truths by speaking directly to mankind. These truths have been handed down to us by means of Sacred Scripture and Sacred Tradition.
God has given us the gift of reason so that we can learn about Him. But because our reason is limited (and God is unlimited and infinite), reason by itself is insufficient. God wanted us to know Him better than by learning about Him indirectly. Just as one can learn a certain amount about an artist by studying his works, but could learn much more about the artist by talking to him, so too can we learn about God by reason, but we will learn much more by listening to God Himself.
There are many truths about God which we cannot understand by reason alone, such as the Trinity and the Incarnation. However, God Himself has revealed these truths to us through our faith, and as a result, we can have the confidence that these mysteries, which are beyond our understanding, are definitely true. The truths we can attain by the use of our reason alone do not have this same safeguard. Indeed, it is extremely fitting that God has revealed these truths to us because God desires us to participate in eternal and supernatural mysteries that are beyond the grasp, naturally speaking, of the human mind.
Pope Pius IX puts it this way:
God in His infinite goodness has ordained man for a supernatural end, to participation, namely, in the divine goods which altogether surpass the understanding of the human mind, since “eye hath not seen, nor ear heard, neither hath it entered into the heart of man, what things God hath prepared for them that love Him.” (1 Cor. 2:9)(Pius IX, Dei Filius)
It is precisely because God has ordained us for an end beyond our comprehension that He gives us Divine Revelation to help us attain this end. If the truths God has revealed to us on earth are wonderful and glorious beyond our comprehension, how great must be the mysteries that will be revealed to us in Heaven!
Pius IX. Dei Filius. First Vatican Council, Dogmatic Constitution on the Catholic Faith, 1870.