The term living will is used to describe a document that records your wishes concerning medical issues.
One of the most common terms in a living will is the DNR — which stands for do not resuscitate. Perhaps you would like to specify the situations in which you would like the DNR to apply. If you are not breathing on your own and there is no activity in your brain — is that a good time to pull the plug? What about if you are in a coma? Do you wish to be fed — for how long?
Perhaps you want to specify that heroic measures will always be taken to keep you alive.
This is up to you — which is why you are writing the living will.