Intensity is a measure, collected for every point, of the return strength of the laser pulse that generated the point. It is based, in part, on the reflectivity of the object struck by the laser pulse.
Reflectivity is a function of the wavelength used, which is most commonly in the near infrared. The strength of the returns varies with the composition of the surface object reflecting the return.
Intensity is used as an aid in feature detection and extraction, in lidar point classification, and as a substitute for aerial imagery when none is available. Intensity is relative, not quantifiable, therefore you cannot expect the same value off the same target from flight to flight or from elevation to elevation.
If your lidar data includes intensity values, you can make images from them that look something like black-and-white aerial photos. ArcGIS provides the ability to create intensity imagery from lidar data.
For more information on how to create and use intensity data in ArcGIS, see Creating intensity images from lidar in ArcGIS.