Scan Disk Problem

I’d like to know what it means by the statement “Scan disk has restarted 10 times because Windows or another program has been writing to this drive. Quitting some running programs may enable scan disk to finish sooner.”? This notice has appeared on both Standard and Thorough scan. I close all programs and lengthen the time of the screen saver whenever I perform this task before going to disk defragmenter. Please explain to me the meaning of this so I will know waht to do. Thank you.