A writ of certiorari is a legal document requesting that a higher court review a case after it's tried in a lower court. The Supreme Court uses a writ of certiorari for most of the cases it hears.
In Latin, certiorari means "to be made certain." In English law, legal documents (called writs) which instructed a court to have a case reviewed by a higher court traditionally began with the phrase Certiorari volumus, or "We wish to be made certain." Anyone who isn't satisfied with the outcome of a legal case can appeal to the Supreme Court for a writ of certiorari, although it's extremely rare to get one.