Click the control or press I to activate.
The Intensity control, part of the Main Diffraction Spikes group, allows you to increase or decrease the brightness at which the Primary Spike effects are rendered.
Higher Intensity values make the Primary Spikes appear brighter.
When you increase the Intensity, the Primary Spikes may seem to get longer. This is normal, though a bit of an optical illusion because the higher Intensity setting is simply revealing the dim extents of the spikes that are farther away from the highlight.
Note that an image will often look more natural if the spikes are kept a bit subtle by being conservative with the Intensity setting.