Something is true if it accords with reality. In other words a true statement is one which matches the reality it represents. If it is raining and I say, "It is raining." then that is a true statement. When it comes to God, God is the Ultimate Reality and thus the basis for all reality. God knows everything (omniscience). God is truth itself since God knows himself, that is he has the ultimate knowledge (omniscience) of the ultimate reality (God). Therefore God is the Ultimate Truth itself.
All truth comes from God. God himself is true and thus truth itself.
“God is Truth itself” (CCC 215)