Two cels are linked when they share their image and the xy-coordinate. Linked cels looks as follow in the timeline:
When you modify one of these cels, all linked cels are modified. In this way you can propagate the same change into several frames. E.g. If you have a static background, you would prefer linked cels, so you can make changes just in one cel to view the change in the whole animation.
To create linked cels you have to
copy cels
in a continuous layer
(i.e. a layer with the Continuous icon 
).
There is a way to unlink cels using right-click and Unlink option in the timeline:
Unlinked cels will contain their own copy of the image. So now if you modify them the change will not be propagated to other cels.
SEE ALSO