The number of children a parent gives birth to is known as the birth rate. If all individuals in a population have two children then the birth rate will be 2. For example, if half the individuals of the population have 2 children and the other half have 3, then on average the birth rate will be 2.5. The above links describe the growth model and provide several experiments you can do using the included applets to better understand that type of population growth. Follow the links and do the experiments there before returning to the discussion below.
Earth faces is that of over-population by humans.
However, there is now little additional land that can be put into use for food production and our wastes are rapidly polluting the environment. Unless major technological breakthroughs are made soon, we are close to the the Earth's carrying capacity. It is likely we will overshoot the carrying capacity and for the first time in recorded history have massive population decline brought on by disease and starvation. You have seen how sensitive a population's growth is to the its birth rate. The decrease in birth rate needed to avert a disaster is not large, but it is one that everyone need to work together to achieve.