God is a Trinity
The word Trinity does not appear in the Bible but the principle is correct. The word is an attempt to describe the 3 in 1 nature of God. The word is a combination of "Tri", meaning 3, and "Unity", meaning 1, a Tri-unity.
God is at the same time a single being and three beings. The three persons of God, God the Father, God the Son, and the Holy Spirit, are so completely unified in what they do that they act as and are one.
From our experience with human beings we know it isn't possible for us to be so unified. Therefore we can't fully understand how this trinity works. None-the-less the Bible makes it clear that this is true.