I do not understand it. I know it is some kind of problem with Laser Printer. Can someone explain it to me?

Transparancies - are old tech. In the days before video capability was built into windows office, powerPoint, visio and word.

Overhead projectors and transparancies were the way group meetings illustrated printouts made from copies of book pages, other print media. Transparencies were made using lazer sensitive plastic (celulose acetate sheets) which would capture (burn) the image onto the media the text or image. When placed on the bed of the Overhead projector, it could be displayed on a wall or projector screen to make the image visible to small or medium sized groups. Handouts, were usually presented so the attendies could follow along.

Now powerpoint presentations and small LCD/LED projectors can do the same things from laptop presentations.

Another dying business staple bites the dust. Mimeograph, typewriters, slide rules... time marches on.

Actually, it not really obsolete, just not a popular as it once was. Its not as impressive when dealing wih customers and it assumes your customers had one (or you needed to bring one with you). For road warriors, its a complication they no longer need to bring, schlep up and down stairs. Bother with setting up or deal with organizing sheets of media, updating pages everything there is a change.

Transparency
http://en.wikipedia.org/wiki/Transparency_(projection)

Projector
http://en.wikipedia.org/wiki/Overhead_projector