They are large flat pans, often made of cast iron, with rounded cavities in them. Various size pans have varying numbers of the round, semi-globular holes in them.
To use, you put the pan put on top of heat source.
Toy versions of the pans are made for girls to play with, with small holes, they use them over sterno cans.
For more information on the use of the pans, see the entry on Kanom Krok.
Many theories abound about Danish missionaries bringing the idea for such a pan back to Denmark, or taking it to Thailand.