Lens Hoods
Light or sun hitting the lens creates flare which can make your images or parts of them look hazy or have spots or streaks of light on them. Occasionally it can look awesome!.
You put a hat on or hold a hand to shield your eyes so you can see properly. A lens hood does the same. If you are shooting directly into the sun, you may need to hold additional shade over the lens as well.
A hood for telephoto lenses is a must. It adds contrast and depth of colour to the image even if the sun isn't hitting the glass. If you are using a wide angle lens, only a very minimal wide hood is used, otherwise you get vignetting.