What is revelation?
Not to be confused with the Book of Revelation, we are referring to how God reveals himself to humankind.
By natural reason man can know God with certainty, on the basis of his works. But there is another order of knowledge, which man cannot possibly arrive at by his own powers: the order of divine Revelation. Through an utterly free decision, God has revealed himself and given himself to man. This he does by revealing the mystery, his plan of loving goodness, formed from all eternity in Christ, for the benefit of all men. God has fully revealed this plan by sending us his beloved Son, our Lord Jesus Christ, and the Holy Spirit. -- CCC, 50
How does God do this?
The divine plan of Revelation is realized simultaneously "by deeds and words which are intrinsically bound up with each other" and shed light on each other. It involves a specific divine pedagogy: God communicates himself to man gradually. He prepares him to welcome by stages the supernatural Revelation that is to culminate in the person and mission of the incarnate Word, Jesus Christ. --CCC, 53
To read more on the revelation of God online you can click here for a direct link to chapter referencing God's revelation and creation to humankind in the Catechism of the Catholic Church.