Light always travels in a straight lines, called rays.
When something blocks a ray of light there will from a shadow behind it
When only a part of the light gets blocked a penumbra is formed (only a bit shadowy)
When all of the light is blocked an umbra is formed (full shadow)
The last ray of light which can pass by an object is called an edge ray, you need to use these in drawings to determine where and how shadows are formed.