Here, thread 1 spawns a second thread. Threads 1 and 2 execute in the same address space, so if thread 1 writes something in this address space, later thread 2 can read it and retrieve what the first thread wrote. In this code, thread 2 will not do anything until it observes the effect of the write by thread 1. Then it executes the rest of the code which will result in the number 1 being printed.

In summary, communication between threads happens by reading and writing shared variables.