DocumentationMathematicaFront EndMenu CommandsCell MenuCell Properties
Cell Active toggles a cell between active and inactive state.
Palette buttons within active cells carry out their script when clicked, while buttons inside inactive cells can be edited.
When a cell is active, the letter "A" appears in the cell bracket.
This command toggles the option Active between True and False on the selected cell objects.
See The Mathematica Book: Section 1.3.6.