How does the birth rate affect the size of population?

The birth rate is the number of children born in a year as a share of total population. Birth rates have important short and long-term effects. Essentially, a higher birth rate will increase the population in the short-term simply because more children are born. More importantly, it will increase population in the long-run because there will be more children in a generation, and when those children grow up there will be more people who are having children, resulting in exponential population growth.