Also called "Matte film", anti-glare film glass is one of two popular treatments the glass surface of a pen display. (The other is called etched glass).
This film is affixed to the glass underneath with some optically clear adhesive.
Anti-glare film provides two benefits
It reduces glare and reflections due to light hitting the tablet. The film does this my scattering that light.
It adds some texture which helps the pen grip better - without this the pen would feel slippery on the glass
As a film, it provides some protection to the glass underneath.
Some films are designed can be removed and replaced. So if you've scratched up the film, you can restore the table to its original feeling.
For SOME tablets, these films are intended to be removed by the user. Some manufacturers even sell replacement films. These films, tend to be easily removed.
For SOME tablets, these films are quite firmly attached to the glass and are not easily removed.
If you remove the film, you'll notice typically that the display will look brighter and the colors more vibrant, however:
you'll notice glare
And of course the glass is no longer protected from scratches
Sometime people confuse these films with lamination. They are very different and have different benefits:
These films are on TOP of the glass
Lamination means that the glass is bonded UNDERNEATH to the display panel
So it is quite possible to have a pen display that has BOTH a film AND is laminated.