To add "Advertisement" above your ads, just visit your Mediavine Dashboard > Settings > Optimize for Core Web Vitals.
Make sure the Optimize Ads for CLS setting is enabled to activate the ad box. Then you will have the option to add the advertisement label.
Do I need to turn this on?
Nope! The label is completely optional, and it is disabled by default. It doesn't have any impact on your earnings, and is strictly an aesthetic choice.