On the chart you can find number of days on the horizontal axis and amount of cards completed for that number of days on the vertical one.
There are 2 red lines on the chart. They are percentiles for the 50% and 85% (you can change that levels by chart options). They mean that 50% (or 85%) of tasks were completed faster than 3 (or 15) days.
So you can provide that information to you customer to predict how many time you need to complete a new task. You can forecast that you finish it in 15 days with 85% probability