As the question states, there are 100 robins in the area of 10 square miles. The density of the robins can be calculated by divided the total robin population (which is 100) by total area (which is 10 square miles). This means there are 10 robins per square mile ( = 100 robins/10 square miles). Using the same approach, if the question provided the density of robins in a given area, say 100 robins per square mile and the total area is also given (say 10 square miles); we could have calculated the total population of robins by multiplying the density with area. In that scenario, the total population of the robins would have been 1000 (= 100 robins per square mile x 10 square mile).
Thus, using the relationship between density, area and population; we can calculate any one of these parameters given the other two parameters.
Hope this helps.