This object can be the border of a frame, a graphic or a table. The icon will only be visible if a graphic, table, object or frame has been selected.
To apply a particular type of border to a single cell, position the cursor in the cell, open the Border toolbar and select a border. Whenever you insert graphics or tables, they already have a complete border. To remove that border, select the graphic object or the entire table and click the no border icon on the Border toolbar.
Further information can be found in the Help in Borders. You can also find information on how to format a text table with the Borders icon.