The umbra is the part of a shadow cast by an object in which the light source is completely obscured.
For point light sources, there is no shadow other than the umbra. For light sources larger than a point, there are places which are part in shadow - i.e. places where the object obscures some, but not all of the light source. These lesser parts of the shadow are called the penumbra and the antumbra.
Assuming your monitor is a light source: Hold a small pad of paper up very close to your eye, so that it covers your view of your monitor. Your eye is now in the umbral shadow of the paper.
The umbra of the moon on the Earth is the area where a total solar eclipse can be seen. The umbra of the Earth on the moon causes a total lunar eclipse.