An ad page is a page that contains the ad snippet (ie a bit of code that defines the content of the ad).
You can insert an ad page (ie an ad) via:
Because the ads are stored as page, they are by default added to the index.
You need to set a configuration to prevent it. See ComboStrap Ads - Global Configuration
This example was done with Google Ad Sense but all ad network will work the same way and provides you at the end a ad snippet.
<html>
<script async src="https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js"></script>
<ins class="adsbygoogle"
style="display:block; text-align:center;"
data-ad-layout="in-article"
data-ad-format="fluid"
data-ad-client="ca-pub-1481679052079799"
data-ad-slot="8776594059"></ins>
<script>
(adsbygoogle = window.adsbygoogle || []).push({});
</script>
</html>