There are four major parts to the self-evident truths spoken of in the Declaration.
First, there is the idea that all "men" are created free and equal. This was meant to show that monarchy and hierarchy did not make sense.
Second, there was the idea that all men were given certain "unalienable rights" by God. In the Declaration, these rights are enumerated as the rights to "Life, Liberty and the pursuit of Happiness."
Third, there was the idea that government had to rule with the consent of the people.
Finally, there was the idea that any government that did not rule by the consent of the governed and protect the rights of the people was liable to be overthrown by rebellion.
These ideas were taken directly from the writings of Enlightenment thinkers like John Locke. By using these ideas that were already quite well-known in Europe, the colonists were able to justify their rebellion against England.