I’m sure most of us never even noticed the presence or absence of letter ‘e’ in the words lightening and lightning. The two words are different because of letter ‘e’ only.

Lightening means to reduce the weight of something and make it lighter
Regular practice helps in lightening the burden of studies during exams.
He believes in lightening the burden of people around him.

Lightning refers to the electric discharge in the clouds
There was huge thunder and lightning last night.
Lightning struck many buildings causing great damage to the city.