When you’re shooting towards a bright light source, like the sun, light may hit the lens directly. When that happens, flare will inevitably appear in your photos. Flare introduces strange reflections and robs contrast from your images. Lens hoods control flare by shading the lens.
But if you don’t have a hood or it isn’t working at the angle you need, use your hand. Raise your left hand to block the light source and shade your lens. You may have to hold your hand at arms length to get it out of the frame. On SLRs and DSLRs you can see the flare in the viewfinder so you’ll know when you’ve got it right. On point and shoots you can see the effect in the LCD.