The Trinity is a theological term to describe God as three persons in one. The actual term is never mentioned in the Bible, but the concept is in the Old and the New Testament.
In the very first chapter of the Bible, God says, “Let us make man in our image.”
Who do you think “us” and “our” are? There is no indication of God being married, so it’s not His wife. And Angels don’t create, they’re a creation of God. There’s only one logical answer: It’s God the Father, Son, and Holy Spirit.
When Jesus the Son was baptized, God’s Word tells us God the Father spoke from Heaven and said, “This is my son. I’m proud of Him.” Then God the Holy Spirit descended upon Him in the form of a dove. All three were there. All three are one God.
It’s a mystery our finite human minds have a hard time grasping. But the Trinity–God as three in one–fills us with wonder at the unexplainable greatness of God.