Initialization Cell makes a cell auto-evaluate whenever the notebook is opened.
When a cell is an initialization cell, a vertical tick mark (|) appears in the cell bracket.
The Initialization Cell command toggles the option InitializationCell between True and False on the selected Cell objects.
The option InitializationCellLoading, set at the notebook level, determines when the initialization cells in a notebook are evaluated.
InitializationCellLoading True evaluates initialization cells when the notebook is opened.
InitializationCellLoading False tells Mathematica not to evaluate initialization cells.
InitializationCellLoading Automatic loads the initialization cells only when you first do an evaluation within the notebook.
The option InitializationCellWarning determines whether you get a dialog box confirming that you want to evaluate the initialization cells, at the time determined by InitializationCellLoading. A dialog box will appear if InitializationCellWarning is set to True at either the notebook or global level. This provides a level of security, because an individual notebook cannot cause initialization cells to be loaded silently, unless InitializationCellWarning is set to False at the global level.
You can evaluate the initialization cells manually at any time by choosing Evaluate Initialization from the Evaluation submenu of the Kernel menu.