Population policy is any kind of government policy that is designed to somehow regulate or control the rate of population growth. It includes attempts to control birth rates, death rates, and immigration rates---the three major factors that affect population growth. It can also include policies designed to regulate the demographics of population, such as the age distribution or the proportion of different ethnic groups. Many countries have such a policy, but its precise form varies substantially.
First of all, some countries are trying to decrease population growth, while others are trying to increase it. This is because it is generally believed that a rate of population growth between about 0% and 2% is most desirable, while less than 0% or greater than 2% is harmful. Many European countries are trying to increase their population growth, while many Asian and African countries are trying to reduce it.
But more importantly, population policy varies dramatically in its strength, from very mild, benign policies like subsidized sex education and public healthcare to severe, draconian policies like mass deportation or mandated abortion. China's "One Child" policy is an example of the more strict end of the scale, while at the more benign end, Jordan's population policy is mainly focused around improving healthcare to reduce infant and maternal mortality.
At the most extreme, even genocide could be considered a radical and violent form of population policy, as despite its horrific means, its goal is the same as much population policy: to remove certain demographics from the national population.
The term "population policy" refers only to the goal---regulating population growth---rather than the methods by which that goal is achieved.