With a multimeter you measure the voltage drop at the resistors. At each resistor you measure a voltage of 4.5 volts.
In the circuit, the voltage of the battery (9V) completely drops off at the resistors (This is the direct consequence of Kirchhoff's Voltage Law (KVL)). Since the two resistors are equal, at each half of the voltage (4.5 V) drops.
The implication of this is perhaps surprising at first: It does not matter which resistors you use. As long as they have the same resistance you measure a voltage of 4.5 volts each.
What happens to the voltages when the resistors are not equal?