I am supposed to simulate this circuit (regulated power supply 5V), however I am not understanding what's the point of the resistor R. Is it a load resistor? Cause i thought these needed to be before the regulator.
If you look at a typical 7805 datasheet you'll find a statement like
Note: Minimum load current for regulation is 5 mA
If the circuit is expected to maintain a 5V output with a load going right down to zero, the resistor R ensures that enough current is drawn from the 7805 to guarantee that the 5mA minimum is met. In this case a 1000 Ohm resistor would be needed.
It's often not seen in the real world because the 7805 will normally be built into a circuit that will always take at least the minimum load.