Oh task_id is the Task ID of step 2.
Basically the idea is, you run your code once (lets call it debugging / programming), that run creates a task in the system, the task stores the environment definition and the arguments used. Then you can clone that Task and launch it on another machine using the Agent (that basically will setup the environment based on the Task definition and will run your code with the new arguments). The Pipeline is basically doing that for you (i.e. cloning a task changing parameters and enqueuing the task for execution).
This means the base_task_id parameter is the Task that the step will clone and enqueue for execution. Make sense ?