Plastic fastener clips are used for attaching LED strips to rough or porous surfaces, and surfaces which otherwise don't "take" to the adhesive backing of LED strips.
The inside measurement is approximately 10mm. The clips fit nicely over 10mm non-waterproof LED strips. They can also be used with 8mm non-waterproof strips but there will be some space between the clips and the strip.
These are recommended for installations where the strips need to be mounted from behind and for attaching to wood surfaces.