Four independent tasks, each taking 1 "second". On the left, one worker does them one after another. On the right, four workers each take one. Press start and watch the clocks.
The point: 4 tasks × 1s = 4s in serial, but ~1s in parallel — a 4× speedup. This only works because the 4 tasks are independent. When they depend on each other, you can't do this — that's Amdahl's Law, coming in Session 08.