Here’s an app note about PSRR of LDO from Microchip. Link here (PDF)
The Power Supply Rejection Ratio is the ability of a device, such as a Low Dropout Voltage regulator, to reject the various perturbations that can be found in its input supply rail by providing a greatly attenuated signal at the output. Generally, the main source of the perturbation will be the output ripple of the DC/DC converters that typically power LDOs.
High PSRR LDOs are recommended for powering line ripple sensitive devices such as: RF applications, ADCs/DACs, FPGAs, MPUs, and audio applications.
One important clarification must be made: PSRR is NOT the same with output noise. PSRR is a measure of rejection. It shows what the part will output based on the given input.