Are you talking about the spotlight entity or texture lights?
For the entity, the inner radius is the angle of the bright part of the light, and the outer radius is the angle of the light in general. The inner radius is always smaller than the outer radius (I never got round to testing what would happen if you did them the other way), and everything inside the inner radius circle will be maximum brightness. Between the edges of the inner circle and the outer circle the brightness will fade.
I'm having trouble explaining what I mean without an MS Paint diagram, and I don't have MS Paint, so this picture should help.
Normally the surface that the light is cast on will be about perpendicular to the direction of the light beam, but this picture shows it as being roughly parallel. Notice how in the middle is noticeably brighter than the rest of the light, well that would be the inner radius. In GoldSrc iirc the light gently fades between the inner radius and the outer radius as opposed to that picture where the outer radius is of uniform brightness.
This picture shows the light being cast perpendicular to the surface. The bright part in the middle is the inner radius, and the less bright part is the outer radius.
The name inner/outer radius implies this value is the radius of the circle of light, but it is actually the angle between the opposite sides of the light cone, in degrees.
Sorry if I'm over explaining myself, I'm just used to being able to provide diagrams, and I'm bad at descriptions.