The Declaration of Independence has two main parts. In the first, Jefferson explains his theory of a just government. In the second, he explains how (in his opinion) the British government has fallen short of this ideal. Because it has fallen short of the ideal, the American people have the right to declare independence.
Jefferson says three main things about an ideal government. First, it must get its “just powers from the consent of the governed.” That is, the people must consent to be ruled and should have a say in how they are ruled. Second, people have the innate right to “life, liberty, and the pursuit of happiness” and governments are created in order “to secure these rights.” Finally, if a government does not secure these rights, “it is the right of the people to alter or abolish it.”
Jefferson then goes on to list a number of ways in which the British government has failed to get its power from the consent of the governed. He lists ways in which it has failed to protect their rights. For these reasons, he says, the American people have the right to overthrow (abolish) the government and create a new government of their own.