A build-taks: which will preload the library and copies the files to the dist-folder.
The default-task: Which first cleans the dist-folder (deletes all files and folders) and then executes the build-task and last but not least deploys it the the abap-stack (detail)
https://answers.sap.com/questions/426165/dist-folder-and-webapp-folder-in-bsp-application-f.html