CellOpen is an option for Cell which specifies whether the contents of a cell should be explicitly displayed.
With CellOpen->False, a small cell bracket is still shown to indicate the presence of a cell.
Cells which are not open can still be evaluated automatically if you set InitializationCell->True.
See The Mathematica Book: Section 2.10.9.
See also: Visible, CellGroupData, ConversionRules.