How does “long, low whistle” involves the characters and events in chapter 5

Johnny uses the whistle to communicate with Pony when he goes off to go to the store while they are in hiding. Gangs use it to communicate.

....heard someone coming up through the dead leaves toward the back of the church, and I ducked inside the door. Then I heard a whistle, long and low, ending in a sudden high note. I knew that whistle well enough.