CellLabel is an option for Cell which gives the label to use for a particular cell.
CellLabel->"" specifies that no label should be used for a cell.
Cell labels are displayed when the setting ShowCellLabels->True is made.
Cell labels are typically generated automatically when cells appear as input or output to the Mathematica kernel.
Cell labels are automatically deleted when a cell is modified if CellLabelAutoDelete->True.
See The Mathematica Book: Section 2.10.3 and Section 2.10.9.
See also: CellTags.