For example in the Assets App - Asset Editor, currently clicking on either button leaves the editor in place.

Clicking on "Cancel" should return the user to the previous state.
If the user had been in Main SubApp and clicked edit to get to the editor, then clicking Cancel should return user to Main SubApp.
If the user had been in view mode for the item and clicked edit to get to the editor, then clicking Cancel should return user to view mode.

Clicking on "Save Changes" should switch the subapp to the view mode of the item, if view mode is implemented.
If view mode is not implemented, clicking on "Save Changes" should return user to Main SubApp.