If you click the link icon on two layers, those two layers will be linked together. That means that when you change anything regarding timing in one of the layers, the other will automatically follow with the same kind of change. This is good for layers that belong together, like a body and it’s head separated out on two layers. So let’s say you need more frames for an expression on the head, so you add a few drawings to it, then the linked body will get the same amount of drawings added to keep the action in sync later down the timeline.
You can link as many layers together as you need. Including annotations. This way your notes “stick” to the drawings you made them for.