In the Bible, the writer of Hebrews described Faith as "The assurance of things hoped for, the conviction of things not seen. (Hebrews 11:1)." From this one sentence, we can see that faith is something that acts on our minds. It's a way of knowing something. It gives us a certainty of something else, beyond what we would ordinarily perceive. You might describe it as a sixth sense. Any of our other senses help us to take in information, and when we take them in we know something. If I look outside in the morning and see a blanket of white flakes on the ground, I know that it's cold enough to form frost. The rest of my mind then suggests that I'll have to scrape my windshield.
In the Gospel of John, John the Baptist is using his sixth sense of faith to know something about Jesus. Looking at Him with his eyes, John could see a man with tan skin, Middle Eastern features, and the muscles of a carpenter. But with the sense of faith, John could see "The Lamb of God who takes away the sins of the world (John 1:29)."
Faith is also one of the Theological Virtues, or Supernatural Virtues of Faith, Hope, and Love. Supernatural Virtues are a gift from God, and they enliven, elevate, and perfect the other virtues. Supernatural virtues also act upon the ordinary faculties of any human being. Faith primarily effects a person's reasoning, or understanding. With faith we are able to perceive and understand things that we wouldn't otherwise. By faith we perceive Christ present in the Eucharist and in the community gathered around it. By faith we understand Scripture to be "The Word of God that contains all things necessary for our salvation (BCP pg. 526)." By faith we understand Jesus Christ to be perfectly human and perfectly God. When we perceive and understand such things, it has a profound impact on how we live our lives.