Lens flare commonly occurs when shooting in the general direction bright light sources, whether they are a part of the image or not. The bright light creates internal reflections and scattering of light inside the lens, which results in artifacts or haze in the final image. A common scenario for producing lens flare is shooting into the sun.
In the above video, Karl Taylor shows us examples of lens flare and how to use a lens hood to avoid it. The lens hood serves the dual purpose of protecting the end of the lens in the field.