This chart allows you to visualize / calculate compound interest, also called cumulative interest or cumulative growth.
Let's say you deposit $1000 to a bank that gives you 5% interest per year; with a normal interest (not compound) you would have: $1050 after first year, then $1100, $1150, $1200; with compound interest you will have $1050 after first year, then $1102, $1157, 1215 ...
The compound growth can be a huge deal because (for above example) in 30 years you would get $4321.94 compared to $2500 without compound interest.
Notice how without compound the growth is steady (straight line) and with compound interest the growth multiplies itself over the years.
Chart can also be used to calculate any income that increases a certain percent each period (day/month/year/etc).