In the Resource Monitor of Windows 7, what does the "disk queue length" really mean?

Disk queue length usually means the length (in number of I/O operations) of the queue of pending operations for a given disk. In other words, how many requests have stacked up waiting for the disk drive to be able to service them. As an example, if I can only process one request a second, but I get 6 requests in a given second, then my queue length will initially be 5 (one request is processed immediately, the five remaining must be queued).

For the meanings of the lines by color, look to the left section at the grey bar towards the middle of the top of the screen. It indicates the color and instantaneous value for each of the variables being graphed.

This article seems to indicate "Disk Queue Length" refers to the # of processes waiting on the disk. My comment on Slartibartfast's answer is probably wrong.