The first ecumenical council was called to settle the Arian controversy — the teaching that the Son of God was the highest of creatures, but a creature still, not eternal and not fully God.
The bishops answered by confessing the Son as homoousios, of one substance with the Father: 'true God of true God, begotten, not made.' They produced the original form of the Nicene Creed and set a common date for Easter.
Nicaea is treated in full on the Councils reference page. As an event, it marks the moment the Church, newly free, used its freedom to confess Christ with a precision that would hold against every later distortion.