Third Party Ads
This is a feature offered by some Email Service Providers (ESPs).
3rd party ads are a way to monetize your email marketing through real-time display ads within your emails.
This is an email monetization strategy (especially if you have a large email list).
This works very similar to how Adsense works for websites:
Enable 3rd party ads through your ESP
Place the ad merge tags within your email (banner ad, body copy etc.)
Targeted ads will be served in real-time to each subscriber automatically (i..e each subscriber will see an ad that is relevant to them)