Here's how Windows Insider chief Gabriel Aul describes the Bug Bash:

With this new build today for PC and Mobile – we're officially kicking off the Bug Bash with Windows Insiders we mentioned last week. Over the course of the next 4 days, we will be publishing multiple Quests inside Feedback Hub that will highlight different areas of the product each day. We will be looking for feedback on each Quest, but you can also just use the parts of Windows and our apps you are most passionate about – the key is just to work together now through Sunday to get the best bug reports possible!

The Quests that you will see for the Bug Bash are not traditional Quests that list steps of trying a feature or scenario out. Many of them are open ended intentionally so that you can perform the steps that come naturally to you in order to finish the Quest and give us feedback on it. Many of them also list more than one scenario that can be tried out to finish the Quest. All the Quests for the Bug Bash will lead back to Feedback Hub so that you can upvote existing feedback or give new feedback based on your experience of completing the Quest. Please make sure to search for existing feedback to upvote before entering a new report.