How is this machine used? (for, I think, peeling the skin from the tomatoes)
That's a manually operated Food Mill/Ricer/Puree-Maker.


The Wikipedia Article for Food Mill shows a pic of the same model you have and from that article:


  Uses of a food mill include removing the seeds from cooked tomatoes,
  removing pulp or larger pieces from foods (creating apple jelly or any
  type of purée), and making mashed potatoes or
  spätzle.


It's not likely that you could uses this for peeling the skin from tomatoes though.