Population density is loosely defined as the number of organisms in an area divided by the amount of area. Population density is typically measured in kilometers squared. High population density can cause increased competition for resources while low population density can cause problems with finding mates and inbreeding.
In studying population ecology a common measurement that is made of a population in a particular area is what is known as population density and that is simply what is the number of organisms in a particular area divided by that area. So you'll typically be getting number of say 0ne butts in a square kilometer or could be the number of peri-kitts in a square mile. Now this gets a little bit complicated when you're talking about aquatic populations because then you have to figure out volume but the idea is the same.
Now population densities can vary from one species to the next at which density they tend to do best but a general trend that you will see is that if your density gets too high then you start having problems in that population in terms of there's a greater chance in disease and greater competition for resources whereas if the density of the population gets too low then you start having problems with finding mates and that can ultimately lead to problems with in breeding for example in some species or some populations. In general you will tend to see that organisms that do really well at reproducing, you will typically find them in higher densities obviously then the organisms that typically have fewer offspring but often each individual offspring is given more resources and those organisms called key selectors are typically found in lower populations, lower density populations.