The surface is the data. E.g. the ridership for Lafayette Street is the upper line of the green area, minus the lower line of the green area. This is called an area chart and is used to show cumulative data and how each part contributes to the whole. In this case it shows the total NYC ridership increasing from ~7000 to ~18000 (units are unclear!), while e.g. the Central Park West ridership has remained approximately constant.