Sikuli is an automation tool which is an internal or College project from massachusetts institute of technology (MIT).
Sikuli is used for automating the web browser or desktop application by using its image , as sikuli is written on image recognition algorithm which accepts images from the client and performs the given operation.
Some of the automation tools like selenium,codedUI etc which automates browser using its control properties like Xpath , Control Id , ControlName , Css Selector , ClassName etc , in certain scenarios like Extjs application which renders its controls or div elements in run time , it will be very difficult to automate using codedUI or Selenium , in this scenario we will go for sikuli automation which can be done easily just with the help of images given by the client or end user. The image given to perform operation like Click , double click , Hover etc, sikuli accepts the image and checks those images in the given Url , once the image found on the page it perform the given operation otherwise through the exception saying "Cannot find the image".
Hope this helps you in understanding the usage of Sikuli.