What are "inalienable" rights?

The Declaration of Independence says, "We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness." But what does it mean to say that our rights to life, liberty, and property are "inalienable" or "unalienable"? If that means that the state cannot take away a person's rights, then does the state violate the rights of criminals by imprisoning or executing them?