Line editors have extremely primitive input and output methods. Typically users enter data by typing terse commands on a text-only terminal. Commands and data, and corresponding output from the editor, will scroll up from the bottom of the screen in the order that they are entered or printed. There is no cursor to move around and select portions of the document being worked on, generally line numbers or a search based context are used to specify which part of the document is to be viewed or edited. Naturally most edits are a line-at-a-time.
Now considered extremely old-fashioned, they stem from the days when a computer operator would be sitting in front of a teletype (essentially a printer with a keyboard), so there was no screen and no way to move a cursor around a document.
Line editors are still used non-interactively in shell scripts.