Ghulūw (excess) means assigning someone a station higher than the true station that belongs to them. Love and veneration cease to be balanced when they attribute a rank or quality that is not rightly theirs.
An example of excess concerning the Messenger of Allah ﷺ is to declare a prophet God or the son of God. The extraordinary rank of prophethood does not remove the prophet's servanthood or place him at the rank of divinity.
Honoring prophethood is rightful; belief in the prophet's divinity is excess.