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The main idea here is replacement level fertility—the average number of children a woman would need to have to exactly replace the parents in the population, assuming no net migration and a stable population. In high-income countries, child mortality is low, but not zero, and there are slightly more boys born than girls (and higher male mortality at older ages). Because of these factors, the number is a bit above two. So the typical replacement level is about 2.1 children per woman.

Why that works: if everyone survived to reproduce and births were perfectly balanced by sex, exactly 2 children would replace the parents. The extra 0.1 accounts for children who don’t survive to reproductive age and the sex-ratio dynamics. The other values don’t fit well: 3.0 would mean a growing population purely from fertility, 1.0 would mean a shrinking population, and 2.5 ignores the mortality and sex-ratio adjustments that inflate the simple 2.0 replacement figure in real populations.