On the first page, the top graph shows how college readiness, as determined by SAT scores, has changed over time. The bottom graphs break out the college readiness rates by various student groups. You can also change the year of the breakout graphs on the bottom. Hovering over the graphs shows more information. All of these graphs are representing cohorts of students, and cohort refers to a group of students who entered high school in 9th grade at the same time and graduated four years later.
The second page contains the same information, but allows you to compare two different graphs.