Misskey can display ads in the instance timeline and other places if enabled by the instance administrator. You can reduce how often an ad is shown by clicking in the upper right corner of it.
If the total number of ads being shown on the instance is small then an ad may still be shown to you often even after you choose to reduce how often it is displayed.
Configuring Ads (Admins)
You can add, edit, and delete ads from the control panel of your instance. In there you can set any image, destination URL, shape, and display percentage for the ad.