Why is totality during lunar eclipse not altogether dark?

Answer 1

Earth's atmosphere refracts some light, mostly red light, into its shadow. This light falls on the moon's surface during a lunar eclipse, turning it red. If Earth did not have an atmosphere, the moon would go completely dark during a lunar eclipse.

Answer 2

The area of the total eclipse is no more than a few hundred miles across, depending on the exact geometry of the eclipse. So there are bright sunlit areas not too far away.

And the Moon isn't big enough to cover the WHOLE Sun. It covers the actual disk of the Sun itself, but the Sun has an atmosphere of sorts, which is plenty hot enough to glow. We call this the "corona". We don't generally notice it, because the Sun is so much brighter than the corona, but with the Sun itself covered, the corona is plenty bright enough to see.

Answer 3

Earth's atmosphere bends some sunlight into the umbra.