The graph represents your equity against villain's range.
let's say you raise from MP w 55 and the BB calls. Assume he does this with the top 20% of his range.
The flop is 763r. Your average equity at this point is 63.5%.
The graphs show where this equity comes from.
You see that your equity is extremely high for ~16% of your hands. These are the times you hit your gutshot, and it doesn't matter much which of villain's 20% he actually holds.
The other 84% of the time, you're not hitting your straight, so the graph reflects how that holds up against villain's range.
At 20%, Villain's range is basically pocket pairs and broadways. For the latter, you're ahead, and villain has about 6 outs, so you have around 75% equity. According to the graph, that reflects the range between 16% and 65%, or almost half the time.
If villain has a PP, he's ahead of you with everything except 22 and 44, and so your equity there is low.
Overall, the graph shows you to be way ahead about two thirds of the time and way behind a third of the time, as shown by the very steep breakpoint at 65%.
So the graph gives you quite a bit more information about your equity than the vanilla average number of 65%.
The graphs are also explained here: