Great question! A denizen is an inhabitant or frequent visitor to a location, so it's intentionally vague. It could include Firstborn (oracle included) or any Ironlanders you might encounter in that region. In the latter case, I'd use an Ironsworn oracle such as Character Role.
It could also be locals from specific settlements. For instance, if there's a steading called Redwick closeby, then wardens and travelers from Redwick are probably a common sighting, whereas Ironlanders from Broken Shield, a steading upriver in a neighboring region, may only travel through occasionally.
The region could be on the path to a sacred site, with Iron Priests passing through on pilgrimage. Or, you may need to keep watch for threats like Raiders, legendary beasts rumored to lair nearby, or Horrors which have been appearing with increasing frequency in recent days.
The Denizens matrix is a flexible, custom oracle suited to each region, and depends heavily on the fiction of your Ironlands. The instructions also suggest "matrix interactions" — tying certain results to other matrices or oracles to produce unexpected results. For example:
You're pressing on, trying to reach Redwick before nightfall, and see a figure's silhouette on the ridge. It's probably a warden, another traveler, or even one of the firstborn, but you roll on the matrix to find out for sure. You roll 83: Rare, and decide to write in "Firstborn" and then roll on the oracle as you call out to the stranger. You get a 48—the stranger's silhouette suddenly straightens to almost giant-height, its long limbs unmistakable—it's a troll! What is it doing so far from the Flooded Lands? ...
I hope that helps!