What do you understand by the sub-replacement fertility in terms of demography?

When the fertility rate of a population falls below the replacement rate which leads to a decrease in the population of a certain group of people, it is referred to as sub-replacement fertility. This happens when the adults of a population are not able to produce enough children to replace them. When the replacement fertility rate falls below 2.1 children per woman, it is said to have touched the sub-replacement fertility rate. The replacement rate is not the same everywhere and differs widely between the developed and developing world countries.