It depends on which part of the site you are on, and the answer has confused a few review sites, so here is the whole picture.
The public calculators and info pages: no ads
The debt snowball calculator, the debt avalanche calculator, the side by side comparison, the velocity banking and DTI calculators, the blog and every help article are ad free. There is nothing to dismiss and nothing loading in the background.
Free member accounts: a few display ads
If you create a free account and use the planner, you will see a small number of display ads inside the member area. Those ads are what keep the free tier free. They are served by Google, and like any Google ad unit they use Google's own cookies, which may include personalized ads based on your Google ad settings. You can turn ad personalization off in your Google account, or use a browser ad blocker; the site will keep working either way.
What Undebt.it itself does is simpler. The site runs no tracking pixels of its own, uses cookieless, privacy first analytics instead of an advertising tracker, and never sells or shares member information with advertisers or anyone else. Your balances and payment history are never part of what an ad network sees. The privacy policy spells out exactly what is collected.
Undebt.it+ members: no ads anywhere
Removing the ads is one of the Undebt.it+ features. Plus is $10 a year, not per month, and there is a 30 day trial you can start without entering a card, so you can see the ad free member area before deciding.
If you ever see an ad that looks inappropriate or misleading, use the contact form and tell me where it appeared. I would rather lose the impression than run something that does not belong on a debt payoff site.