Birth rate is the amount of children being born in a certain time period, usually measured on a "per-woman" basis. 2.1 children per woman is considered a "magic number" for the birthrate, as this is the rate at which a population will remain roughly constant, barring immigration (the additional .1 children is for those who die young). This statistic is often examined as part of political economy.
This measurement is usually applied to a country, or a specific ethnic group.
An overly high birth rate can lead to social, political, and/or environmental problems.
A low birth rate can mean bankruptcy of the economy if the aging population is very much larger than the upcoming next generation. This is especially true when the high cost of medicine for senior citizens is factored in.