The setting helps you to indicated key threshold or milestone that was achieved on the way of the target completion. It is define as percent of target level and achieved percent on the chart is colored in orange when it is above given Threshold and below Target.
In the same manner as target label its appearance on the chart might be customized. By default its label is ‘Tresh’ but it might be renamed to whatever make more sense for the purpose for which it was used e.g. Milestone.
Please check below how it is shown on the chart.