Answer:
Purpose of the statement.
Explanation:
An infinitive phrase is one that has an infinitive verb (one that begins with "to") and has elements that complete its meaning.
In the case of the above excerpt, the infinitive phrase "In order to form a more perfect Union, establish Justice, insure domestic Tranquility ..." represents the purpose of the United States declaration.
A "purpose" represents the expression that explains why something is being done or created. For example: What is the purpose of a graph? The fork is designed to help us eat, so its purpose is to help us eat.
So the infinitive phrase in the excerpt shows why the US declaration was made. According to the infinitive phrase, the statement was made to form a more perfect Union, establish justice, ensure domestic tranquility, provide common defense, promote general welfare, and secure the Blessings of Freedom for us and our posterity, order and establish this Constitution for the United States of America.