The reference colorspace is the one that every colorspace defined in the config converts from (or to). It may or may not be defined in the config. If it does, the to_reference and from_reference transforms for the reference colorspace are necessarily null.
As for the purpose of having a reference colorspace, one cannot simply take a bunch of pixel values and transform them to colorspace X without knowing the colorspace associated with the incoming pixel values -- a transformation requires well-defined source and destination colorspaces.
Within ocio, a conversion from colorspace X to colorspace Y is built by concatenating colorspace X's from_reference and colorspace Y's to_reference*.
The alternative to having a reference colorspace would be for the config to define transformations from each colorspace in the config to every other colorspace in the config, which I think is N! entities instead of N.
The ACES 1.0.3 config uses ACES-2065 as its reference colorspace, but a custom config could as easily define all of it's colorspaces in terms of an ACEScg reference or some camera vendor's encoding and gamut.
*most ocio colorspaces are invertible so they need only provide a from_ or a to_.
Hoping this is helpful.