Etched glass is one of two popular treatments the glass surface of a pen display. (The other is called an anti-glare film).
"Etched glass" describes a microscopic texture applied to a glass surface.
Etched glass provides two benefits
It reduces glare and reflections due to light hitting the tablet. Etched glass does this my scattering that light.
Provides more "grip" for the pen. Without the etching a pen would feel "slippery" on the smooth glass surface.
Like other anti-glare treatments, etched glass can produce an effect called anti-glare sparkle which is a "rainbow noise" effect. Depending on the tablet there will be a little or lot of this sparkle. Learn more here: Anti-glare sparkle.
Etched glass is a permanent treatment to the glass. It cannot be removed.