It's just the values from the sensor, which is a (mostly) linear counter. The different photosites on a Bayer sensor have different colored filters, and the value for each site represents the light which gets through that filter. The name "RAW" is meant to convey precisely that the values are simply that "uncooked" reading.
In a sense, then, the RAW file is in the camera's native colorspace, where the primaries match whatever happens to be the wavelengths of the filter. Converting to a reference color space like sRGB or Adobe RGB is a matter of transforming from the native space to that, and to do it correctly you need to know the particular properties of the filters in a given camera's sensor.
Raw converters use a color matrix (specific for each camera model) to get their results. You can see this in the dcraw source code — it's my understanding that author Dave Coffin gets most of this information from Adobe, who in turn has relationships directly with
the camera manufacturers.