What dose whenever any form of government becomes destructive of these ends, it is the right of the people to alter or to abolish it. mean

This means that the American people is allowed to take back power from the government if it acts against human safety, liberty, and justice ("these ends" in the text).

The full quote from the Declaration of Independence goes as follows:

“That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness.”

It has to do with a principle called "consent of the governed," which established the legitimacy of governance: the government's ultimate goal should always be the protection of natural rights or justice in order to be legitimate.

a social contract.

"a social contract is an agreement between citizens and their leader where the citizens give up certain freedoms in exchange for protection. the constitution of the united states is an example of a social contract."