What is a derailment?
A derailment occurs when you're not accomplishing your goal at the rate you've committed to. This happens when your datapoints veer off the yellow side of the graph into the white. See how the top left portion of the graph below is faintly yellow, while the bottom right is plain white?
Datapoints that are off the bright red line will be colored red (see the highlighted datapoint in the graph below). If you end the day with a red datapoint, your goal will derail when your countdown ends at the goal's deadline!