My parents bought one for their computer at home years ago, they are supposed to protect your eyes from the glare. Kind of like sunglasses. You may be able to find one at any of the usual computer stores, maybe online as well.
I used to have one on my computer at work. I think the idea was protecting my eyes when sitting in front of a comp for 8 hours at a time. I just found it kind of annoying and felt like it was always in the way. The privacy screens are kind of nice though.