Why PSK stands for Phase Shift Keying, a digital modulation method that carries data by shifting the phase of the carrier rather than changing its amplitude or frequency. In amateur PSK31, for example, a phase reversal represents one data state and no reversal represents the other, which lets a very narrow 31 Hz signal carry keyboard-to-keyboard text. Because the amplitude stays essentially constant, PSK works well with modest power and in weak-signal conditions.
Watch out Pulse Shift Keying sounds plausible but is not a real mode name, and the choices mentioning packet or power confuse PSK with packet radio or with amplitude-based keying.
The P in PSK is Phase, the same P as in PSK31, the phase-shifting keyboard mode.