Why do I get a message to input my iCloud password every time I log on to my iMac?

Every time I log on to my iMac I get this message "Please enter my password for my email address for iCloud. If I enter my password nothing happens, so I just cancel out and nothing happens. Is there anyway to stop this message from appearing? I have iMac 21.5 inch, using OS Yosemite 10.10.5, originally purchased in 2011, Processor 2.5 ghtz, Intel core i5, Memory 4 Gigs 1333 MHz DDR3 See Image below. Thank you in advance.