Light generally travels in a straight line. We can use this property of light to determine the umbra, penumbra, and antumbra part of a shadow by drawing lines from the edges of the light source toward and past that of the occluding object.
Flat-Earthers regard the lines in an eclipse diagram as a violation of the fact that a light source emits light in every direction. In reality, the lines are there only to determine the boundaries of the shadow’s regions in an eclipse diagram.
In the shadow’s umbra, an observer will see the entire light source obstructs the occluding object. In the penumbra, they will see a part of the light source blocked. And in the antumbra, they will see the occluding object obstructs the inner part of the light source.
These apply to all shadow, not only in the case of eclipses. And these also apply to all objects, not just the Sun, Earth, & Moon.
In the case of a solar eclipse, if an observer is inside the umbra, then a total solar eclipse occurs. If they are in the penumbra, a partial eclipse occurs. And if they are in the antumbra, they will observe an annular solar eclipse. These can be determined by drawing straight lines between the sun as the light source, and the moon as the occluding object.
References
- Umbra, penumbra and antumbra – Wikipedia