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Bode phase plot shows phase shift ∠G(jω) vs log ω. Each zero contributes +90°, each pole -90°; the phase approaches these asymptotic values away from corner frequencies. Phase lag (negative) indicates lag; phase lead (positive) indicates lead. Phase determines stability and transient overshoot.
From your prerequisite on Bode magnitude plots, you know how to sketch the gain |G(jω)| as a function of frequency using asymptotic approximations. The Bode phase plot is the companion diagram: it shows the phase angle ∠G(jω) — how much the output signal is shifted in time relative to the input at each frequency — as a function of log ω. Together, the two plots fully characterize the frequency response of a linear system.
The phase contribution of each pole and zero follows a simple pattern. A first-order zero at s = −z contributes a phase that transitions from 0° (well below the corner frequency ω = z) to +90° (well above it), passing through +45° exactly at the corner frequency. A first-order pole at s = −p contributes the mirror image: from 0° down to −90°, passing through −45° at the corner frequency ω = p. The transition region spans roughly one decade below to one decade above the corner frequency. A system's total phase is the sum of contributions from all its poles and zeros, so you can sketch the phase plot by superimposing these individual S-shaped transitions on a log-frequency axis — exactly the same superposition logic you used for the magnitude plot.
Phase lag (negative phase) is the normal condition for most physical systems: output lags behind input. The more poles a system has, the more total phase lag it accumulates at high frequencies. A system with n poles and no zeros approaches −90n° as ω → ∞. This phase accumulation matters critically for feedback control: the phase margin is the amount of additional phase lag the system can tolerate before going unstable at the gain crossover frequency (where the magnitude hits 0 dB). If you read −160° of phase at gain crossover, your phase margin is 20°. Typical design targets are phase margins of 45°–60°, which correspond to good transient responses without excessive oscillation.
Phase lead (positive phase) is less common in open-loop systems but is deliberately introduced by lead compensators — controller elements with a zero closer to the origin than their pole. By adding positive phase near the gain crossover frequency, a lead compensator improves the phase margin without dramatically changing the gain crossover location. The phase plot is your map for this design work: you read off the current phase at the crossover frequency, calculate the phase margin deficit, and then design a compensator to contribute enough positive phase to close the gap. This connection — between the phase plot and stability margins and then between stability margins and time-domain performance — is why the Bode phase plot is not just a mathematical exercise but a practical design instrument.