Specify the shell script to be executed when the execution of the task is selected. This is where the actual work happens. When the script executes, the current directory will be the workspace directory, just like the shell script builder. So depending on your SCM, you might have to cd into a directory. Try pwd and ls to find out whether you need that or not.
Environment variables from the latest build are available to the task script. In addition, TASK_ID is set to the identifier for this task run ("build#-task#"), and HUDSON_USER is set to the user who started the task, if it was manually triggered.