Definition of merger
"Merger" in English means "merge" in French. merger applied to poker means "to swing one's range". By "balance" here we mean "balance". A player who is going to "balance his range" is a player who will have the worry of being impossible to read. For example, on the river, a balanced player will want to have bluffs and very strong hands when betting or raising all in. If a player is not thrown and bluffs too much on the river or not enough, he will be easy to read and the more experienced players will have no difficulty playing against him. If, on the contrary, he is thrown, he will be impossible to read.
Is it really necessary to have merger of his range?
If you play games at very high limits or if you always play against the same excellent players, yes, it will be important to have a good merger your rank so that it is not readable. On the other hand, if you are playing against amateur players who have a lot of weaknesses, you will not need merger your rank; you will have to exploit the weaknesses of your opponents. For example, you are on the river in position and it is your turn to play. If you're up against a very conservative player who lays down too easily, bluff him often! If you have a good hand, bet small! Conversely, against a player who folds too much, don't bluff and bet bigger only your good hands.