Disk I/O High Activity
This seems to be related to your earlier question about High Disk IO. Have you looked at other items to see what is going on as suggested in my answer there? Wait Stats, Query Information, Etc? I think you should look at everything. And in this case it still looks like your disk queue length on that drive at that one time isn't bad. You need to really look at your IO Latency as I mentioned in that answer. It looks to me like you are still hunting around for an IO issue, but should look at the full set of information as I mention in the answer there to really help you out.

But the question here: This screen is in the Resource Monitor, and the blue line refers to the active time of the drive. So in this case you had a quick spike of higher activity on the K drive and mostly the drive wasn't very active.