What feature is that?
The ability to see what ads a specific Facebook page is running, even if you aren’t in the targeted audience.
Why is this important?
It’s all about transparency. You can learn more about a page and what they’re doing.
There are limitations though. Right now, you have to be in Canada, and will only see ads being shown in Canada, on the device you’re currently using (so if you look on a desktop but the ad is for mobile viewers only, you won’t see it). You also can’t see offer ads or some dynamic ads.
This feature became available in Canada in Nov 2017, and is scheduled to become available in the US in summer 2018, with special information being available for election-related ads.