what does it mean in the Declaration of independence when it says "governments are instituted among men to secure these rights?"

This refers to the rebounding aspect of social contract - - on the government's behalf of course - - in which the citizens have the power to overthrow or replace a government if it misuses its powers, breaking the social contract.

This quote in particular states that it is the government's duty to protect the rights of the people - - rather than taking advantage of the people.