The antumbra is a part of a shadow cast by an object. As seen from the antumbra, the perimeter of the light source is visible, but some of the inside of the light source is obscured.

For an object to cast an antumbral shadow, the light source must be larger than the object.

Assuming your monitor is a light source: Hold a small pad of paper between your eye and your monitor so that it covers the middle of the screen, but you can see all the edge of the screen. Your eye is now in the antumbral shadow of the paper.

The antumbra of the moon on the Earth is the area where an annular solar eclipse can be seen.