When something is spread, it's stretched or extended as far as it can go. It's a beautiful sight to see a bald eagle spread its wings and fly.
When you spread a picnic blanket on the ground, you open it and stretch it across the grass. You can also see the ocean spread out in front of you, or spread butter on toast, for example. Any way you use it, spread implies an extension across some distance, a stretching of some sort. The expression spread too thin means you are having trouble finding the time and energy to do everything in your life.