Some notes on the curves:
1. DC biasing point - when there is no signal of interest, this is the amount the current through BJT , and Vo2 is the voltage at Vout. Choice of Vo2 is by design - affected by Vdd available, amount of signal gain required, quiescent power dissipation by BJT, linearity required...
2. Base current deviate from DC biasing point following the signal of interest
3. Base current changes will cause collector current to change according to curves shown as Ib1, Ib2, Ib3..., the current will cause voltage changes on Vout, choice of Rc will affect the magnitude of the Vout signal.
4. Non-linear region of IV curve (Cut-Off region) - if Vout swing were to reach this area, the amplification of signal will be distorted. This will cause clipping on output signals, THD (3rd order harmonic distortion)…
Additional note about BJT:
1. BJT is actually Current Controlled Current Source - Base current controls Collector (compare this to Mosfet - Voltage Controlled Current Source – Gate to Source voltage controls Drain current)
2. Rc will determine the signal gain, DC biasing point and some other parameters.
3. From Kirchhoff law (V, I)
a. only interception of each Ib curve with the load line is possible for a given circuit
b. so as Ib changes, the instantaneous Vc (collector voltage) will be the intersections of load line with that instantaneous Ib curve.