He mentions the laws of nature (1), he states that all men are created equal and that they're endowed with unalienable rights (2), and that governments derive their power from the consent of the governed (3). These are principles that could be derived by reason alone. They're foreign to the ways of tradition which believed in the divine right of kings, aristocracy, and a Christian God, not a god of nature.
The central idea is the same as the main idea. Most of the time, the main idea is mentioned in the Title. Without reading the essay, I do not know what the central/main idea is.
