Finale orphan here. Getting used to Dorico but today this appeared and I have no idea what it is. I don’t know what to call it so I can’t search help. Anyone?
That’s the System Track, which shows by default and which can be hidden using the command in the View menu or by pressing Alt/Opt. T. It’s handy for many functions (selection, deletion, creating an editing block location, etc.) which can best be looked up in the manual. Now you know what it’s called. 
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It’s part of the System Track (the grey bar at the top), showing that a particular region is selected.
Why it’s in the middle of your staff; I’m not sure.
TBH, I usually turn the System Track off, as you don’t really need it, unless you’re copying large chunks of music from one place to another, and need to make sure that you’ve selected everything.
Thanks. So many mysteries . . .
It looks as though the OP either has the vertical distance between systems set to a very low number or has done manual editing.
Yes, this way the system track of the second stave overlaps with the first stave. Should be gone in galley view.
Hmm. What’s an OP? (is that me?) This is a new flow today so I didn’t set any vertical spacing. Does the system assign that somehow or inherit it from an earlier flow?
In this case, yes, it stands for “original poster”.
That’s the insert mode stop bar.
Scroll down in the manual just after that section for info on deleting it.
Thank you.