Each bar shows two quantities for a given ranking position. The full bar length is the number of ballots that included at least that ranking. The blue portion is how many of those ballots had that ranking actually consulted by the counting algorithm to help elect a winner.
A large blue portion at a lower ranking means that ranking was important: many voters needed it to contribute to the result. A large gray portion means those ballots had the ranking available but it was not needed — the voter’s higher choices already determined the outcome.
This chart is useful for understanding how many ranking columns a real ballot needs to provide. If many ballots are using the 5th or 6th ranking, a 3-column limit would have left those voters without an effective voice.