Editable is an option for boxes, cells and notebooks which specifies whether their contents can be edited interactively using the front end.
Even with the setting Editable->False, the contents of an object can still be copied as a whole.
Editable is an option for InterpretationBox and TagBox, as well as for StyleBox.
Setting Editable->False effectively allows you to "write protect" elements of notebooks.
See The Mathematica Book: Section 2.8.10 and Section 2.10.9.
See also: CellEditDuplicate, Selectable, Copyable, Protected.