Explaining property title issues to an idiot

Can someone please walk me through this whole "unregistered owner" situation? I’m struggling to wrap my head around it. Are they actually considered the owners of the property? What does that even mean in practice? And more importantly, can anyone actually challenge their claim to the unit? I tried digging into this myself, but beyond the fact that the last person officially on the deed can technically sell the place via an unregistered party, I’m stuck. Legal jargon isn't exactly my strong suit... I feel a bit out of my depth here.

Thanks for not judging me for being a complete novice at this.

Look, lawyers don't get this, so don't sweat it. There are endless possibilities here, literally,
meaning anything can happen...
Take today, for example: the county assessor is trying to sync everything up with the official property deeds, and the whole goal is to make sure the parcel maps match what’s actually written in the land registry records.
But it wasn't always like this. After WWII, things got messy, and the biggest headache was all that communal property legislation—that's largely what caused this entire clusterfuck. Plus, you have the old-school way of doing things where people gave their word on land deals and didn't bother hiring an attorney for every little thing.
Now, you have some people who rushed to fix their titles right after the turn of the century, and then you have others living in houses that belonged to their great-great-grandparents. That becomes a nightmare if the ancestor had five kids, and those kids had three more, and they had two more... suddenly you're looking at 25 different heirs you have to sue just to prove you own the house you've "lived in forever"—which is what people always say in disbelief once they realize how many people they have to take to court, like an eighth cousin living in Australia for forty years.
In your case, though, the situation is different because you're actually listed as the owner. It’s possible you inherited it through a probate ruling from a grandmother or mother, but you don't actually "possess" the true interest. For instance, imagine 100 years ago, a grandmother and her sister agreed to sell the place to their neighbor, Joe. The grandmother passed away, leaving only your mother (let's call her Mary 🙂), while the aunt had no kids. Even though there was a deal, neighbor Joe never officially recorded the deed because he was too busy. Eventually, your mother was listed on the title, and she passed it down to you, so now you're the one on paper. But the reality? The actual land is being enjoyed by Joe's grandson—Steve! Now Steve and his wife are growing tomatoes or whatever out there, and they've been there since they can remember, ever since Grandpa Joe skipped over his son, Blaze, as the heir because Blaze drank too much and Joe just handed everything to his grandson Steve via a will—it's called unrecorded ownership. Basically, he has a piece of paper showing he got it from your grandmother and her sister, but since it was never filed with the county, he was never "on title." That doesn't mean he isn't the de facto owner.
I hope I've cleared up some of the terminology. Since I don't know the specifics of your exact situation, I can't give a more precise comment.