This is the number of children who die expressed as a proportion of all the children who could possibly die. For example, if there are 1000 liveborn children and 10 die, the mortality rate for these children is 10 per 1000.
It is more useful to know the mortality rate than only the number of children who die. Therefore, it is not good enough to simply know how many children died. Mortality rates allow you to compare deaths between different-sized groups of children.