You know when the sun is high in the sky, just a bit forward of you, and you're trying to see into the distance, and there's that annoying glare coming from the sun - so you instinctively put your hand up to your brow to block out the sun and it helps you see much more clearly? Or shooting at night, when looking at a scene, and just off to the left is this hugely annoying floodlight on a roof, causing the most miserable glare, and you put your hand up to block it out so you can see better? That's what a lens hood is doing for your lens!