Copyable is an option for Cell which specifies whether a cell can be copied interactively using the front end.
Even with the setting Copyable->False, the expression corresponding to a cell can still be read into the kernel using NotebookRead.
With Copyable->False set at the notebook level, no cells in the notebook can be copied interactively in the front end.
See The Mathematica Book: Section 2.10.9.
See also: Selectable, ReadProtected.