To say a country’s birth rate is increasing is to mean it is rising in numbers. This would mean that the country’s population would be larger as the numbers go up.
This means that population is increasing in younger people. This means there would be a lot of those people. Think of the population pyramid, and how, lets say, that there are more 30-50 age people then 50-100 or 0-30. This really means if the birth rate increases, that there will be more population, and more population of that age.
Here's an example that's somewhat close to my answer:
