I don't think there's a particular unit of measurement, just 256 "steps" between lowest and highest for each of red, green and blue.
Take a look at the Wikipedia page for Adobe RGB - it has some useful diagrams. In particular, the diagram showing "The CIE 1931 xy chromaticity diagram showing the primaries of the Adobe RGB (1998) color space" - the three points of the triangle (the "primaries") correspond to these points (moving anti-clockwise, from the right):
- R = 255, G = 0, B = 0
- R = 0, G = 255, B = 0
- R = 0, G = 0, B = 255
The same system of 256 steps is used for other colour spaces like sRGB, even though they may well be a different size to Adobe RGB's colour space.