As well as knowing God by our natural reason, we can also know Him from Supernatural Revelation. The word “revelation” comes from the Latin, meaning “to draw back the veil”. By revelation, God draws back the veil that hides Him from us.
God has often revealed Himself to men through means beyond the ordinary course of nature. This type of revelation is supernatural (i.e. above nature), or Divine Revelation. This is opposed to the natural revelation that God makes of Himself in the external world and the revelation He makes through our natural reason and conscience.
Some of the truths that God reveals are beyond the power of human understanding. This means that we could never (by our own ability) have known these truths if God had not revealed them. For example, we could never have known about the Blessed Trinity if God had not revealed it to us.
Divine Revelation comes down to us by two means: through Sacred Scripture (the Bible), written down under divine inspiration and through Tradition, handed down orally from Apostolic times (and contained in the writings of the Fathers of the Church). We read the Bible with great respect because it is the Word of God. We treat Tradition with as much reverence as we do the Bible because God speaks through Tradition as well. It is wrong to believe the Bible alone without Tradition. That is like believing the Word of God written in the morning and denying it spoken in the afternoon.