What is a penumbra? and what makes it happen?

Answer 1

From Internet:

In a penumbral lunar eclipse, only the more diffuse outer shadow of Earth – the penumbra – falls on the moon’s face.

Answer 2

Umbra is the dark part of a shadow. Penumbra is the lighter part of the shadow. Here is a url to read. You will need to add h ttps:// (without the space) before.
sciencing.com/difference-between-umbra-penumbra-8625669.html