Acrylic slats are used to control the thickness of metal and polymer clays while manually rolling them out. They are placed on either side of the clay--much like railroad tracks--and the roller is run over them until the clay is the same thickness as the slats.
Each color represents a different thickness. Some clay artists use playing cards to measure clay thickness and may refer clay being 1, 2 or more cards thick. The conversion between slats and playing cards is provided for you with this chart.