Every publisher has unsold inventory. A sponsorship ends on the 20th, the next starts on the 1st, and for eleven days a prime slot shows nothing.
A house ad is whatever you put there instead.
Why it matters more than it sounds
Those impressions are real people looking at a real space you control. If the slot does 20,000 impressions a month and sits empty a third of the time, that is 6,600 impressions of prime placement going to waste every month.
It is also the cheapest inventory you will ever have, because it costs you nothing.
What to put there, best first
1. Your newsletter. Almost always the highest-value use. An email subscriber is worth far more over time than a single ad impression, and the audience is already engaged with your subject. A clean "get the weekly guide" unit routinely outperforms paid banners on click rate.
2. Your own product. If you sell anything — a course, a book, templates, a service — this is free promotion in your best position.
3. Your best affiliate offer. Pick the one that already converts and put it in the slot. Real revenue rather than a placeholder.
4. "Advertise here". The lowest-yield option but a real one: a small unit linking to your advertise page. It sells the inventory it occupies, and it signals to visiting brands that sponsorship is available.
Rotate rather than picking one
You do not have to choose. Run the newsletter unit at 60%, your product at 30%, "advertise here" at 10%. Weighted rotation costs nothing to configure and covers all three purposes.
The rule that makes it safe
A house ad must never compete with a paid campaign. It fills the slot only when nothing else is eligible.
In practice this means giving house ads the lowest priority so they can only win a slot no paid campaign wanted. Get this wrong and you are giving away inventory a sponsor paid for — which is both revenue lost and a delivery shortfall you will have to explain.
Track them like real ads
House ads earn nothing directly, so it is tempting not to measure them. Measure them anyway.
Knowing your newsletter unit gets a 1.4% click rate and your product unit gets 0.3% tells you what to run more of. It also gives you a benchmark for what your inventory can do with genuinely relevant creative — useful evidence when a sponsor's banner underperforms and you want to suggest a different one.
What it does to your fill rate
With house ads configured, fill rate goes to 100% by definition. There is always something to show.
That changes what you report. Rather than "20,000 requests, 13,000 filled", you have 20,000 filled impressions of which some were yours. Be clear with sponsors that their figure counts only their own delivery — never let a house impression flatter a sponsor's number.