I call it a light valance or light rail. (Ours just went on today....) Ours is nailed to the outside of the cabinet, since it is a decorative piece of molding. I think it's more common to have a piece of wood attached on the bottom inside edge of the cabinet.

As Vickimp says, it's called "light rail" and it's attached to the bottom of the cabinet. It not only hides undercabinet lights, but it also adds a finishing touch to your cabinets.

Here's the design of ours (bottom pic):

Thank you thank you. I DID think it was called light rail, just couldn't find in Search.

The ones I was given are not decorative, as in shaped, but straight stock in an L shape, 2 1/4"x2"x3/4". I've been assuming the second side is designed to further hide the light source.

I only have 6' sections. Will cut a piece and clamp under the cabinets to see what the effect is. Add'l thoughts on this will be appreciated.

Rosie

If you have face-frame cabinets, there is already a 1" recess across the bottom. Frameless folk have a flush bottom panel, so some form of light rail is requisite. If you use an L-shape, you either must make it a taller molding to get the same valance effect (as a non-L-shape), or place the lighting behind the L strip. Either an L or a single molding can be attached to frameless, but L is screwed on from under, while single strips have to be screwed in from the inside of the cabinet, with the resulting exposed screw heads (there is the possibility of just nailing with a few small finish nails and glue).
If this is DIY and you're painting or staining to match, consider using a hardwood molding, as this location is subject to some rough handling; banging with blenders, dishes, etc, and pine may dent or splinter.
Casey