operating-point analysis
Before a circuit can amplify or switch anything, it has to settle into a steady resting state with the power on but no signal applied, like an idling engine waiting at the lights. The operating-point analysis finds exactly that resting state: the steady DC voltage at every node and the steady current in every branch.
SPICE does this by treating capacitors as open circuits (no DC current flows through a fully charged capacitor) and inductors as plain wires (a steady current sees no inductor), then solving the resulting DC equations. The command is '.op'. The result is often called the bias point or, for a transistor, the Q-point. For example, with a transistor biased so its base sits at 3 V, a 0.6 V base-emitter drop puts the emitter at 2.4 V; across a 4.7 kilohm emitter resistor that sets about 0.5 mA, and SPICE reports exactly that collector current and the matching collector voltage.
The operating point is the foundation every other analysis stands on. AC and noise analyses both begin by finding this point, then study small wiggles around it. If your transistor's Q-point is wrong, the amplifier built on it is wrong, so checking the .op results, asking whether the nodes are where you expect, is the first thing a careful designer does once the circuit converges.
Run '.op' on a voltage-divider-biased transistor stage and SPICE prints the base, emitter, and collector voltages plus the collector current, letting you confirm at a glance that the device is biased in its active region and not slammed against a rail.
The DC resting state: every node voltage and branch current with power on but no signal.
The operating point assumes everything has settled and nothing is changing. Capacitors are treated as open and inductors as short, which is only true once all transients have died away.