Lightening is often used when the word should be lightning. It is a mistake that is easy to skim over when checking your own writing.

Lightning is a flash of light in the sky caused by electrical discharge. (It can also be used to mean fast and sudden.)

Lightening is the -ing form of lighten. Lighten has two meanings as a verb:

1. to make less dark, to make or become lighter or brighter, to shine/glow.
2. to make or become less heavy or burdensome, or to make or become more cheerful or less serious.

It is possible that some confusion arises because lighten can be used to mean to flash when referring to lightning (but I think this is rare).