Text of Amendment:
The enumeration in the Constitution, of certain rights, shall not be construed to deny or disparage others retained by the people.
Why This Amendment Exists:
When the Bill of Rights was first proposed, the major argument against it was that by specifying some rights that the government was not free to violate, there would be the implication that the government was free to violate any rights not specifically protected in the Constitution. The Ninth Amendment was written to address this concern.
Of all the amendments in the Bill of Rights, none is stranger or harder to interpret than the Ninth. At the time it was proposed, there was no mechanism by which the Bill of Rights could be enforced. The Supreme Court had not yet established the power to strike down unconstitutional legislation, and it was not widely expected to. The Bill of Rights was, in other words, unenforceable. So what would an enforceable Ninth Amendment look like?
Strict Constructionism and the Ninth Amendment:
There are multiple schools of thought on this issue. Supreme Court justices who belong to the strict constructionist school of interpretation essentially say that the Ninth Amendment is too vague to have any binding authority. They push it aside as a historical curiosity, in much the same way that more modernist justices sometimes push the Second Amendment aside.
But most justices do believe that the Ninth Amendment has binding authority, and they use it to protect implicit rights hinted at but not explicated elsewhere in the Constitution. Implicit rights include both the right to privacy outlined in Griswold v. Connecticut (1965), but also basic unspecified rights such as the right to travel and the right to the presumption of innocence.