Choropleth maps allow you to visually represent changes in a given data variable across a particular layer using color. To make one, go to the Styles bubble icon in the green toolbar to the upper left of the Layer Editor screen. From here you'll see that Color Progressions (Choropleth) is the third option in the tab. Select the attribute you'd like to represent from the dropdown menu and from their set your stop values and colors to determine which segment of the spectrum of your data set will correspond to each color in your color ramp. Also, you can choose from a variety of color ramps, as well as re-balance your stop values with the click of a button!