What is a thread? A thread is like a mini-program within a program that can execute at the same time as other threads.
What happens when you use multiple threads? A program with multiple threads, then, can do multiple things at once.
What label begins a thread in Scratch? In Scratch, any block whose label begins with "when" essentially demarks the start of a thread; think of what Scratch calls a "script" as a thread.
Why do you think you should use separate threads?It's often helpful to use separate threads for conceptually distinct tasks.