Details on role of "ground" electrode?

Can anyone give (or link to) a clear explanation of the mechanism behind the functional role of “ground” electrode in an EEG electrode array? All answers I’ve found by internet searches are frustratingly vague (e.g. “for common mode rejection”), without explaining the mechanism behind how it functions. Help on this would be much appreciated.

Brandon, hi.

Here is a thread on the OpenBCI Forum,

With a link to a TI app note.