Questions moderation can be enabled/disabled in the Activity management module -> Engagement settings -> Moderation settings.
If this option is enabled, all of the submitted questions have to be approved by the moderator (or speaker or event planner) to be published and visible in the app.
In the app, the question management feature looks like the screenshot below. You can choose to approve or reject a question, and you can also modify it.
Approved questions will be moved to the published tab, while the rejected ones will be moved to the rejected tab.