A lens flare is phenomen of circle glow and/or sparkle on photography caused by internal light reflection inside a lens components. As light bounces around inside the lens elements, it creates multiple reflections of the light source and pretty streaks and glows. This is in basic negative phenomen but in some cases can make a nice effects on photography. Photographing a scene with Sun on it will test your lens immunity for lens flare problem.
Example of the Lens flare (D)effect