If something is unchallenged, that means no one is questioning or opposing it. If none of your classmates raises an objection when your school introduces a new dress code, you can describe the rules as unchallenged.
If you notice unchallenged prejudice in the way teams are chosen in gym class, you might want to speak up — or you might decided to lay low and leave the situation unchallenged. In politics, if one person runs for an office with no opponent, they are also unchallenged: "I'm the unchallenged candidate for class treasurer, so my name is the only name on the ballot."