Stabilizers are the metal-and-plastic mechanism under longer keys — space bar, shift, enter, backspace. On almost every stock mechanical keyboard, they rattle. The rattle is the loudest, most annoying sound the keyboard makes, and most people put up with it because they do not know it can be fixed cheaply in an afternoon.
Why they rattle
A stabilizer is a small wire connecting two plastic stems that travel up and down with the key cap. When you press the key, the stems and wire move together to keep the cap level even when you press at one end. The metal wire moves against bare plastic, which produces a sharp ticking sound. Cheap stabilizers also have a loose fit, so the wire bounces against the housing — that is the rattle.
What lubing involves
You take the keyboard apart enough to access the stabilizers (on most modern boards, the case opens with a screwdriver). You remove the stabilizers, clip a small tail off each stem with nail clippers (the "clip mod"), apply a tiny amount of dielectric grease to the moving parts and a small bead to each end of the wire where it touches the housing, and reassemble.
The total time is around an hour for a tenkeyless board, mostly spent figuring out which screws to undo. Once you have done it on one keyboard, it takes twenty minutes on the next.
What changes
The space bar goes from a sharp clattery thump to a deeper, quieter, more satisfying sound. The shift and enter keys stop announcing themselves. The keyboard sounds noticeably more expensive. People who have not seen what you did will assume you bought a different keyboard.
This is genuinely the highest impact-to-effort upgrade in the hobby. A pot of dielectric grease costs about ten euros and lasts for dozens of keyboards.
What to skip
Lubing the switches themselves is also possible but takes hours and the difference is subtle compared to stabilizers. If you have hot-swap switches and want a smoother feel, buying pre-lubed switches is much less painful than lubing each one yourself. Stabilizers, though, are worth doing every time.