"Greyscale" brackets all over the place

Hi there! Done several LibreOffice Writer documents and in all of them these “greyscale” brackets pop up in various places. For example, one I’m looking at right now: “]100[.” I have not found [admittedly, I’m new to this, so . . .] any help searching on my own.
What are they?
Did I do something “wrong”?
How do I get rid of them? [assuming I need to get rid of them]
Thanks!

They are bookmarks or cross-referenced items, if you delete them then the bookmarks or cross-references won’t work.

You can turn off visibility: Click Tools > Options > LibreOffice Writer > Formatting Aids and untick Bookmarks.
You can search Help for “bookmarks”, or refer to the Writer Guide, download from English documentation | LibreOffice Documentation - LibreOffice User Guides