I have a closet in the basement where when I did the vinyl plank floor, I ran out so the planks don't quite go to the back of the closet all the way. Problem? Yes? A bit ugly? Yes. But in reality the problem is 100% of the time covered by boxes and anyway I can live a happy life in this house for decades and not be affected. That's 0% tech debt.
On the other hand if my gutters are clogged, there's interest on that because the longer I wait the costlier it will be to deal with since clogged gutters can lead to basement leaks or gutters themselves detaching. Or, if my stoop is broken, that's not just an eye sore but I keep tripping on it, the faster I fix it the sooner I stop tripping. That's a high-interest debt that should be fixed asap.
In engineering, a high-rate debt could be some architectural thing that slows down the development of every single feature. You want to quickly pause features to pay this down so everything can move faster. On the other hand, some file that you never touch having some shitty code or lots of TODOs may be a very low interest debt since in practice you never touch it or are otherwise not bothered by it other than knowing that it's ugly - like my closet floor.
Engineers make two mistakes around this - fixing zero-interest debt when there's more important things to do on one hand. On the other hand, when they say "oh, product management/leadership didn't sponsor our tech debt fixing" - it's often because we fail to articulate the real cost of that problem - explaining that it's high rate and how it's costing you.