A passing threshold is the minimum number of questions a person needs to answer 'correctly' in order to Pass. If a person does not meet the passing threshold, their submission will be marked as Failed.
If your quiz does not have a passing threshold, the default result is always Unevaluated.
To add a passing threshold:
Go to Quiz Settings under the Configure tab.
Enable the Pass or fail candidates switch, and enter the minimum number of correct answers required to pass.
Click on Save changes.