You supply the timer NE555 through the Pin1 (GND) and Pin8 (Vcc) with power. In your experiment, you use a voltage of Vcc = 9 V.
Between the two terminals of the power supply, there are three identical resistors which function as a voltage divider and provide the following voltages...
These voltages provide the reference voltages for the trigger and threshold input. You come back to these voltages in the experiments 4 and 5.