Hams sometimes bandy about the term “phase noise,” but few hams really understand it. Most of us can figure out that less phase noise is better than more phase noise, but that’s about as far as our knowledge goes.
To help you understand the concepts, there is a paper about phase noise on www.rfic.co.uk. While there is a lot of complicated, engineering math in the paper (this is complicated stuff, after all), the first two pages offer a simple explanation of the basic concepts. In explaining how phase noise affects a system, the paper notes:
In transmitters local oscillator noise is amplified by the subsequent amplifier stages and is eventually fed to the antenna together with the wanted signal. The wanted signal is therefore surrounded by a band of noise originating from the phase noise of the local oscillator. Therefore the noise generated can spread over several kHz masking nearby lower power stations as shown in figure 3 (shown below).
To relate this to a common example in ham radio, this is why you want rigs with low phase noise at your Field Day operation. Rigs that have poor phase noise performance will seriously raise the noise floor, affecting all of the other radios, especially those attempting to operate on the same band. For example, a 40m phone station running a rig with relatively high phase noise might swamp a station running 40m CW.