A mandolin is used to cut ingredients (fruit, vegetables, etc.) into very even slices or small sticks. It consists of flat bed with an adjustable blade (plus a range of additional different plates or blades) over which the items are pushed to and fro.
If you intend to buy:
Mandolins can be bought from catering suppliers. Professional quality ones are stainless steel, precise, and can be dismantled. They are perfect but rather expensive. Those that you find in the supermarket are generally useless.
The model you can see in the photos is usually sold at trade fairs and markets. It's a good quality tool (made in Germany) that I've been using for years.