'stetched thin' means trying to cover more and more, with the same amount of resources. The resources are stretched thinner and thinner. So if you have a lot of tasks to do, you can only give a small amount of attention to each. The more tasks to do, then more your attention is 'stretched thin' and you can only give a small amount of time to each. Think of trying to make a pizza with the dough getting thinner and thinner as you try to stretch it over a larger and larger area. It's stretched thin.