The answer is immigration rate minus emigration rate.
Emigration is a movement of organisms from the region of settle. So, when organisms leave a population, they emigrate.
Immigration is the opposite process of the emigration in which organism move to some area.
Population change depends on several factors: birth, death, immigration and emigration. If population size is increased, then the death rate must be less than the birth rate. Also, emigration rate must be less than immigration rate. So, to increase population it is necessary for birth and immigration to be higher than death and emigration.