Graduated symbols is one way of visualizing data that can be quantified on a map in GIS. Quantitative data is data that can be express as a quantity, amount, proportion, or range. For example, if you have a database locating all the birds in an area, the data can be quantified to show the total number of birds in a given space, or a density such as the number of birds per square acre. When mapping out the location of birds, the data can be symbolized using graduate symbols which displays the values using shapes of varying sizes. The greater the number of birds in a given area or the higher the density of those birds, the symbols will be larger.
When visualizing GIS data with graduated symbols, the data is categorized into ranges that are then assigned a sized symbol. In this example below, cities in the eastern parts of Canada and the United States have been symbolized using graduate symbols based on 2015 population data. The graduated symbols have been given data ranges based on natural breaks (Jenks) in the population data. Cities with the smallest population values are symbolized with a very small circle. The cities with the largest population sizes are symbolized with the largest circles.
While graduated circles are a popular shape used to visualize quantitative data on a map, they are not the only shape that can be used.
Christopherson, G. L. (2013). Mapping Quantities: Proportional or Graduated Symbols. Lecture.
Buckley, E. (2013). Understanding statistical data for mapping purposes. Esri.