What are these boxes?

Attached is a small table, generated by pasting part of a web page into LibreOffice. The cells of the table are outlined in red. Inside many cells are smaller boxes outlined by thin gray lines. Questions:

  • What are these boxes? I.e. what are they called: paragraphs, text boxes, whatever?
  • How can they be deleted?
  • How can they be created?
    Thanks - jon

The file is available at:

These boxes are cell borders of the table.

If you click in the cell you see a tool bar at the bottom popup. There you can chang the border line to “none” or to another line type (= create it).

This is how I see the sample file with LibreOffice

You probably have View->Text Boundaries unchecked. In this state the boundary lines are not shown. But I want to remove these things from the document not just make them invisible.

That are section boundaries. You can remove them in Format > Sections. The list allows multi-selection, so that you can remove them all in one step.