stable/unstable equilibrium point in inverter with feedback circuit
First circuit: Stable equilibrium point because of negative feedback. An unwanted (random) small increase in V1 will reduce V2 (negative slope) while the feedback line works against it (positive slope) and does not allow such a change.  

Second circuit: Unstable equilibrium point (in the middle) because of positive feedback (two inverter in series). An unwanted (random) small increase in V1 will increase V2 - and the feedback will support this change (also positive slope). The remaining points (latch) fulfill the stability properties (slopes of the curves) as mentioned above (case 1).

Third circuit: Stable equilibrium point (see case 1), however, the circuit will oscillate because we have positive feedback for a single frequency which fulfills Barkhausen´s oscillation condition: 180deg phase shift due to 3 inverters in series and another 180 deg due to propagation delays and capacitive phase shift.

(Comment: In contrast to the comment as given in the figure, I rather would say that the equilibrium point is a stable one (fixed and stable DC values); however, there is a signal swing around this stable point >>> Oscillation. 

Another example: A BJT gain stage has a fixed quiescent (eqilibrium) point -  and the output signal swings around this point. Otherwise the term "equilibrium" would not make any sense.)