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Density
Density is how many annotations are active in each moment of a video. An annotation is active in a moment when it runs across that moment, and not only when it starts there. A long annotation counts in every moment it covers. Coverage tells you how much of the video you have coded. Density tells you where the busy moments are.
Example: You code hand gestures in a 30 minute class. In the first 5 minutes several gestures run at once, while the teacher explains the task. The chart has a high peak there and a flat line after minute 20.
Why it matters in GRAPE MARS
The Timeline Density module draws this as a chart across the video. Its subtitle says "Annotations active over time, stacked by category", so each category has its own band. You can see a peak, a quiet stretch, or an even spread. That helps you pick the minutes to watch first.