Voucher codes can serve various purposes, but the only kind that users frequently see are the "invite codes" used for duels with friends. The invite codes follow the "quizcover:" header. They can be entered in that form, though it's most convenient to just tap on those links.
Some vouchers enable you to bring up a particular question, but if you do so, the points won't count for rankings, streaks, achievements, records, or whatever.
Quizcover uses various third-party modules, and in this section you can view their license terms.
Go there if you're curious who worked hard (some of them for about three years) to bring you Quizcover.
There is a question-related feedback form that you can access via the Post-Solution Screen, but for general feedback, such as ideas for new questions, you can use the form that is accessible via the "Lest We Forget..." section of Settings & More.
Quizcover links to this wiki (also from its home screen), though its makers give you a warning that they don't control the content on this side, so they merely recommend it, but don't provide any guarantees for it.