Green is data being WRITTEN to the drive.

Blue is data being READ from the drive.

Orange shows the impact of a specific task, as chosen by checking a box to highlight the task on the left of the window.

UPDATE:

Many thanks to RoG below for more clarification on this answer:

Green is data being WRITTEN TO and READ FROM the drive.

Blue shows a statistical quantifier of the amount of data that has been written to and from from the drive, as explained by RoG below:

"...Blue...showing the current value of the green line divided by the maximum value of the green line, as a percentage."

Orange shows the impact of a specific task, as chosen by checking a box to highlight the task on the left of the window.