Stools are commonly  used to maintain a cover between top and bottom reinforcements bars. It is used in arrangement of reinforcement bars of slabs. As we are walking on the top reinforcement mesh stools should strength enough to bear the load without changing the gap of two layers.

there are different types of stool shapes available in the constitutions and also different height it depent on the structure
stool used in reinforcement 
stool used in reinforcement

Cover blocks in reinforcement