These are long narrow strips of plastic intended to be inserted into the slots of aluminum extrusions (all sizes, including makerslide). The strips are black in color and do a wonderful job of giving your project or product a finished look.
A popular application for these slot covers is to conceal wires that have been routed down the length of an extrusion. As you can see in the pictures, the slot covers are hollow on the inside and provide ample room for routing.
Slot covers are sold and delivered in 1 meter lengths, but can be easily cut with sharp scissors and placed end to end with a near seamless appearance.