For example, click on the actual object (underneath the populator above it) in the object window. In your node network (to the right, at the bottom) will appear the Object Maker and Density Shader at the bottom. Take a Cloud Shader and plug its output into the Density Shader. Now, the Cloud Shaders' whites and blacks will mask the placement of your object in the scene. Rather than use a Cloud Shader, you could use an Image Shader and create an image of just black and white blotches that will do the same thing. Or a Power Fractal shader can be used in this way.
What it does is "mask" the population by putting the object where there is White and nothing where it is Black.