Translation:The interior bottom of my room is the exterior top of your room.
I recommend checking out the course notes; they discuss the difference between an interior/exterior surfaces.
In short: The interior surfaces are those which you can see or touch from the inside of a space. The exterior ones are the ones you can see or touch from the outside.
The English word "top" can refer to either the interior top ('aqroS) or the exterior top (yor). The English word "bottom" can refer to either the interior bottom (bIS'ub) or the exterior bottom (pIrmuS).
It might help to consider a building with a single room. Its ceiling is its interior top and its floor is the interior bottom. Its roof is its exterior top. Its exterior bottom, I suppose, would be the underside of its foundation.
In a multi-storey apartment building, one person's floor is usually another person's "roof" (using the word a bit loosely).
You might also consider a pair of waterproof boots: If the exterior bottom of your boots gets wet, that's not a problem; that's what they're for! If the interior bottom of your boots get wet, however, you've gonna have a bad time, because that means there's water inside your boots.
I ended up just hitting copy and paste, and switching from Word Bank to Keyboard...
As I kept hitting stuff for it--and it kept telling me I was wrong... and putting up things that I had no clue how they were different from what I wrote. Like, I had literally just wrote what they told me I should write.