When a policeman lays hold of someone and says, 'You are under arrest', reality changes. The person in question had been a free citizen just a moment before. But now he or she is in the custody of the State.
God's words do not merely affect reality, but can create reality. In the book of Genesis, when God says, 'Let there be light', light is instantly created.
John 5 recounts how Jesus met a man who had been crippled for 38 years. The Lord Jesus says, 'Get up, pick up your sleeping-mat and walk', and immediately he is healed and starts walking.
Likewise, at Mass when the priest speaks in the person of Christ saying, 'This is my Body', reality changes. The appearance remains the same but the reality is now the Body and Blood, Soul and Divinity of Jesus Christ. This faith is based on the words of Jesus Himself. (The image is a depiction of the Institution of the Eucharist)
Fr Robert Barron gives some great insights on the Real Presence of Christ in the Eucharist in this clip: