Grand piano desk slides are solid brass guide rails installed on either side of the piano’s music desk.
They allow the music desk to slide smoothly in and out or move across the piano case for convenient positioning.
Polished brass
14" Long. Sold as a set.

DESK SLIDES EXPLAINED:
Grand piano desk slides are the pair of rails or tracks that the music desk sits in. On a grand piano, the music desk — the desk you put your music on — isn't fixed. It slides in and out of the piano in front of the lid prop, on two guides:

On older pianos those guides were wooden grooves lined with felt. On most modern replacements they are brass channels — often sold as "Grand Desk Brass Slides - Pair - 14 inch" to replace the wooden ones. The felt lining inside the grooves is important — it lets the desk slide smoothly without scratching or rattling. When that felt wears out, the desk gets loose or squeaks.

So when you hear a tech talk about desk slides, they mean that track system that lets you: pull the desk forward toward you to read music, and slide it back / remove it completely when you want to close the fallboard.