The growth curve shown above is logistic growth curve or S-shaped curve.
Logistic growth curve is considered more realistic because unlimited resources are not available in an ecosystem or in a habitat, where
$\mathrm{K}$ - stands for carrying capacity.
$\mathrm{N}$ - indicates population density, which is the number of species of a population per unit area.
$r$ - is for intrinsic rate of natural increase.