https://wiki.scn.sap.com/wiki/display/SAPMOB/Best+practices+for+scheduling+device+connections
http://help.sap.com/Download/Multimedia/pdf-afaria7sp14/Device_management.pdf
that option is documented in the manual in the link above.
Randomize by allows you to set the interval for randomizing the start time for the scheduled task.
The Wiki (link above) gives an answer to your question.