Evaluatable is an option for Cell which specifies whether a cell should be used as input to be evaluated by the Mathematica kernel.
With Evaluatable->True, typing Shift-Return in the front end when the cell is selected will cause the contents of the cell to be sent to the Mathematica kernel for evaluation.
Evaluatable is more often set for styles of cells than for individual cells.
See The Mathematica Book: Section 2.10.9.
See also: Evaluator, InitializationCell, CellEvaluationDuplicate.