It goes through each year of the Brazilian GP and plots the lap times’ density. For comparison, the green section is the distribution of lap times across all of the GPs hosted at Brazil. The dashed lines represent the quartiles and the median (the median is the center dashed line). So, farther to the left, the faster the year was, the farther to the right, the slower.
Edit: Additionally, the more washed out the slope is, means the field was spread farther. The tighter (and higher) the peaks are, the closer the field is.