Topical relevance
Topical relevance is how closely a page or link matches the subject at hand. Links and mentions from pages on your topic count for more.
Topical relevance is how closely something matches the subject in question. In SEO it usually comes up with links: a link to your coffee shop from a page about coffee is topically relevant, and one from a page about car insurance isn't. It also applies to pages: how well a page sticks to the topic someone searched.
For a marketer, it's a guide to where to spend effort. One mention on a respected site in your field beats a dozen from sites that have nothing to do with you.
Relevance in links
Search engines look at the page around a link to understand it. A link from a page about brewing equipment, with words that describe your grinder, tells them something useful. A link from an unrelated page tells them little, and lots of them can look like a link scheme.
For example, Fernhill Coffee gets two links in a month. One comes from a well-read home barista blog's review of its espresso blend. The other comes from a general news site's list of local sponsors. The second site is bigger, but the first is the one that says what Fernhill is good for.
Relevance in AI answers
AI platforms work at the level of passages and meaning. When someone asks about the best espresso beans, the platform searches, reads pages about espresso beans, and names the brands those pages discuss. A brand mentioned often on pages about the exact subject is far more likely to come up than one with plenty of links from elsewhere.
So for AI search, topical relevance is less about link strength and more about being discussed in the right places: the reviews, lists and forums that cover your subject.
In AnswerOrbit
Sources shows the pages the platforms read for each of your topics. The Gaps tab lists sources whose answers name your competitors but not you, scored half by how often answers use them and half by how many of your competitors they name. Those are the topically relevant places worth being in.