The image shows two diagrams. The top of the first diagram is vertical column of four circles representing a queue of waiting tasks. Below it is a rectangle representing a thread pool. The thread pool rectangle has three arrows pointing down from it representing three tasks making progress.

The top of the second diagram is a vertical column of four circles representing a queue of blocked threads. Below it is a rectangle representing a semaphore (or any blocking construct). The semaphore rectangle has three arrows pointing down from it representing three threads making progress.

Both diagrams are the same except for what each of the elements (the column of circles, the rectangle, and the arrows) represents.