What is a lens hood used for?

I have a Nikon camera. I bought an extender lens to go with it. The extender lens came with a lens hood. What is its purpose exactly?

The lens hood is ideal if it is threatening to rain, stops raindrops hitting the front element of the lens, it's also designed to stop 'lens flare' when shooting in bright sunlight to the left or right of the camera.

A lens hood is used to reduce possible lens flare caused when light strikes the front element of the lens at an obtuse angle.

Each lens hood is specifically designed for a certain focal length (primary lenses) or range of focal lengths (zoom lenses) to reduce the possibility of lens flare.

The UV filter you have attached to your lens, protects the front element from sticky fingers as well as mud and rain. It is much better to clean a filter than the front element of a lens. Why? Because if you happen to scratch the filter, you can replace if for under $60. A scratched front element of a lens can sometimes cost more to replace than buying a new lens.

Here is what a UV filter did to protect my $800 lens.

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As you can see, all I had to do is replace the lens hood (under $15) and the UV filter (about $60) and I was good to go.