Conservation is the understanding that the quantity of an object stays the same even when it changes in appearance. A concrete operational child between ages seven and eleven understands this fact because of the development of his cognitive ability while a preoperational child does not.
For example, change the tea cup of a 5 year old boy from an oblong shape to a short round shape of the same size, he begins to cry because he believes the change in cup has translated to a change in quantity.
In addition, change the denominations contained in an amount of money owned by each of the children of these two categories, the pre operational child seems a lot bothered, but the concrete operational child has enough knowledge of conservation to be undisturbed by this re-arrangement.