What is this and how do I get rid of it?
We could probably use a bit more context: version of Writer? operating system? Is that at the cursor position? Was an upgrade involved?
To me looks like a text cursor indicating left to right writing mode. Though I’ve never seen it that way, maybe platform dependent.
From pull down, Insert/Object/line?? If it is, Right click on it, backspace or cut. ?
It looks like the “Direct cursor”. To turn it off select Writer’s “Tools” > “Options” > “LibreOffice Writer” > “Formatting Aids” and remove the tick from the “Direct Cursor” box. For what it does, try Writer’s “Help”.