TFR

TFR = total fertility rate. 

TFR is basically the average number of children per lifetime per woman in a population, but statistically modified so that it uses the current birthrate at each age that a woman could have children. So, for example, it uses the 2023 birthrate of 24 years olds, plus the 2023 birthrate for 25 years olds, plus the 2023 birthrate for 26 year olds, etc. 

The long term trend of a place's population (ignoring immigration and emigration) is stable when the TFR is 2.1 (it is 2.1 rather than 2.0, because about 5% of people die before having children, and because slightly more boys are born than girls). Below that, the population is on a path towards decline. Above that, the population is on a path towards "natural growth."  TFR is, roughly speaking, a highly accurate and data based 25 year forecast of the non-immigration based population change (since that is the age range over which the vast majority of women have children) in a place.

The actual change in population depends upon the ages of the women in the country, the current death rate (eventually 100% of people die and the vast majority die after having had all of their kids, so death rates don't impact TFR much), and immigration/emigration. 

Seoul, South Korea has the lowest TFR in the world at about 0.6. It can get up above 8 in parts of Sub-Saharan Africa and Afghanistan. A few decades ago, the U.S. was close to 2.1. Now, every U.S. state has a TFR below 2.