Supply voltage rejection ratio, kSVR (AKA power supply rejection ratio, PSRR), is the ratio of power supply voltage change to output voltage change.
The power voltage affects the bias point of the input differential pair. Because of the inherent mismatches in the input circuitry, changing the bias point changes the offset voltage, which, in turn, changes the output voltage. The real mechanism at work is ΔVOS/ΔVCC±.
In a Texas Instrument data sheet, for a dual supply op amp, kSVR = ΔVCC±/ΔVOS (to get a positive number in dB). The term ΔVCC± means that the plus and minus power supplies are changed symmetrically. For a single supply op amp, kSVR = ΔVDD/ΔVOS (to get a positive number in dB).
Also note that the mechanism that produces kSVR is the same as for CMRR. Therefore, kSVR, as published in the data sheet, is a DC parameter like CMRR; when kSVR is graphed vs. frequency, it falls off as the frequency increases.
Switching power supplies can have noise on the order of 20 kHz to 200 kHz and higher. KSVR is almost zero at these high frequencies, so that noise on the power supply results in noise on the output of the op amp.