what does it mean in the declaration of independence when it says "governments derive their just powers from the consent of the governed?'

In the Declaration Of Independence, this refers to yet another one of the famous John Locke's ideas known as 'Social Contract'

Social Contract is the idea that citizens give a government its power and have the authority to take away that power - - In turn, the government promises to protect the people and their Natural Rights.

Social Contract was created in order to stop a government from misusing its power. If a government was to misuse its power, social contract stated that the people/citizens could replace or overthrow the offensing government.

The Declaration of Independence says, "Governments are instituted among Men, deriving their just powers from the consent of the governed." This means that government gets all its power from the people. The people set up the government. The people run the government.