CellDingbat is an option for Cell which specifies what dingbat to use to emphasize a cell.
The setting CellDingbat->"" displays no dingbat.
Dingbats are placed to the left of the main contents of a cell, aligned with the first line of the contents.
Dingbats are placed outside of any cell frame.
The setting for CellDingbat can be any string. A typical setting is "\[FilledSquare]".
CellDingbat is often set for styles of cells rather than for individual cells.
See The Mathematica Book: Section 2.10.9.
See also: CellFrame, Background.