This illustrates how a diode is OFF until it has enough voltage across it (V_F). Then, once it is ON, it locks in at V_F and any additional voltage goes across the resistor, which sets the current through the circuit.
Play with the applied voltage and watch how the voltage splits across the diode and resistor. Note how the V_F for this diode is about 0.7 V, which is a common PN juntion diode forward voltage.
