Some have tried to explain God as being one like a man and woman are one in marriage. Others see God as some kind of committee of three yet one. But these are not entirely satisfactory. Some have tried to explain the relationship between God and man like this. God is a cube in three dimensional space.
Mankind are squares in a two dimensional plane.
These are the same kinds of conceptual problems that God has in trying to explain Himself to man and man in trying to understand God. The cube must try to explain himself in terms which the squares can understand yet be more than that. The Bible gives us many clues about what God is like. Somehow God is one and yet three. Somehow Jesus is God and man.