These two words, Lightning and Lightening, have very different meanings!
Lightning is a noun or adjective for the powerful flash of electricity that happens during some rainstorms and thunderstorms:
A flash of lightning lit up the darkness of the storm.
As an adjective, lightning means “very fast and brief”:
The kung-fu master had a lightning fast kick.
Lightening on the other hand, is a form of the verb to lighten, which means “to make something lighter” where light can mean color, brightness, weight, or mood (emotion):
The sun caused lightening of her hair color.
The stars faded in the lightening sky of the morning.
Lightening your load will make it easier to carry.
With the happy music, a lightening mood filled the crowd.
It is easy to remember that the suffix -en often means “make” in English. Like soften means “to make softer,” lighten means “to make lighter,” and lightening is what you call that process. Also note that we have the word enlighten, which means “to expand consciousness or increase understanding” (literally, “to fill with light”), and the adjective enlightening often describes something that increases our understanding, such as a book or theory.