they are endowed by their Creator with certain unalienable Rights
The Declaration's second self-evident truth follows from and explains the principle of equality: human beings are born with unalienable rights. "Unalienable" means non-transferable: people can't surrender these rights, even voluntarily. No one is born with a right to sell himself into slavery, or to authorize another person to kill him. But what is a "right"?
A right is a claim that a person may rightfully make against someone who would deprive him of what is his own. If a person owns clothes or books, he has a "right" to them. If someone takes them away, the original owner has a legitimate claim to get them back. We should note also that a right from one point of view is a duty from another. For every person with a right, there are an indefinite number of persons who have a duty to respect that right. Using the example above, the person who took the clothes or books has a duty to return them--or rather he has a duty not to take them in the first place.
People aren't born with clothes and books, of course. They come to own such property either by receiving it as a gift--for instance, from parents or friends--or by working to earn it. But people are born with rights to earn and keep such property. These rights, also called natural rights, are rightful claims to what one owns by birth, or by way of one's nature as a human being. The Declaration goes on specifically to mention three of them.