Lightning is an electric current that exists in thunderstorm clouds.
Within a thundercloud high in the sky there are many small bits of ice (frozen raindrops) that bump into each other as they move around in the air.
All of those collisions create an electric charge.
After a while, the whole cloud fills up with an electrical charges that spread out from top to bottom.
The positive charges or protons form at the top of the cloud and the negative charges or electrons form at the bottom of the cloud.
Since opposites attract, the positive charge is forced to build up on the ground beneath the cloud.
The grounds electrical charge concentrates around anything that sticks up, such as mountains, people, or single trees.
The charge coming up from these points eventually connects with a charge reaching down from the clouds, then a lightning strike is formed.
Lightning works same way as if you rub your feet across carpet and then touched a metal door handle.
The end result is you end up getting shocked!