Shielding - Stops Uplight and Other Wasted Lighting.
Shielding - from Worst, to bad, to less bad to good. A "Full Cutoff" fixture has "Zero Uplight".
Light going up (uplight) is wasted, contributes to skyglow and light pollution. Light that goes horizontal can do the same but also causes glare and possible trespass on neighboring properties (spill light). Glare reduces the effectiveness of lighting as it interferes with the eye's ability to adjust to the illuminated surroundings targeted by the fixture; there is a reason eye level fixtures (lamps) have shades.
Unfortunately, most outdoor area lighting in use today for residential purposes, and often for street and commercial use, are of a Semicutoff style with a bucket shaped plastic lens and bulb that hang below the metal portion of the fixture. Often called barnyard lights, these cheap fixtures can put up to 35% of their light into the sky and are a notorious source of offensive glare and light trespass. It is not uncommon to see them with makeshift shields installed or even painted black.
Modern fixtures have a "BUG" rating that describes the light distribution in terms of Back, Up and Glare lighting. But for lay purposes the Full-Cutoff concept suffices. You can Identify a Full Cutoff fixture by the fact that the light source is somewhat recessed into the fixture. New fixtures installations on state and federal highways are Full-cutoff. This is recognition of the superior lighting provided and the need to address light pollution and trespass. When selecting or buying fixtures ask for those approved by the International Dark Sky Association (IDA).
"BUG" rated fixtures covers the source of the illumination, allowing the eye to more fully adapt to the provided light and target the light to an area that can be effectively illuminated. There is no purpose in throwing light up or out a shallow angles that produce glare and trespass.