Unalienable rights are abilities that people have that can never be taken away from them by a government. In the Declaration of Independence, the US outlines these unalienable rights as the right to "life, liberty, and the pursuit of happiness."
The privileges and immunities clause is supposed to help protect citizens from one state when they go to a different state. This prevents discrimination.
In the Dred Scott case, the Supreme Court ruled that Dred Scott (who is a slave) is considered property. That means he has no legal rights and is not protected by this clause, even though he lived in a free territory with his owner.