An aeroshell is a rigid heat-shielded shell that protects a vehicle from pressure and heat created by drag during atmospheric entry (see blunt body theory), slows it down during entry, and may protect it from debris during spaceflight.
The plaque on the display explains:
"This spacecraft was a section of the Voyager Balloon System which was launched near Rosewell, NM. and landed on White Sands Missile Range. These bright, shiny aeroshells projected an illusion of flying saucers. Aeroshell was designed for slowing down a missile for landing on Mars. This display is believed to be the only one "in captivity". "
Links: kevinrandle.blogspot.com, en.wikipedia.org