What is the use of lens hood on SLR camera?

A lens hood is used to prevent lens flare.

It prevents flare by blocking stray light that might come in from a side angle to the lens. Light that comes in that way can "bounce" off the inside of the lens housing and cause a reflection off the optics.

Some lens hoods are "petaled" which means they have cuts in them that make them look like a flower. This is done, especially for a zoom or wide angle zoom lens, to prevent the hood from blocking the optical angle at which the lens is capable. Otherwise, too long of a lens hood without the "petal" shape and you'd get an image that had the hood in it at the edges.

To prevent lens flare from bright object, usually the sun.

A lens hood is used to stop the sun's rays from reflecting off your lens during day time shooting and stops the possibility of color streaks across your photo.

Hope this answers your question.

JeffD
http://www.photographyscene.com

used to reduce lens flares usually outdoors but under some lighting set ups handy also

It can help reduce flare when taking a shot of a back-lit subject, and helps to keep rain off the surface of the lens. Watch out for vignetting (dark corners) if using a hood on a wide-angle lens though. To check, with the hood attached to the lens and both hands holding the camera, place the tip of a finger on the rim of the hood and look through the viewfinder. If you can see your finger tip, the hood will cause vignetting.