"Clean, Lubricate, Adjust", as Maxwell said. It is usually encountered when discussing Leica camera bodies, those things need a checkup every two decades or so. But, yes, lenses can be CLA'd too.
For lenses, it basically means dismantling the thing, cleaning dust and other deposits from the lens surfaces, replacing the goop that lubricates the focus and aperture mechanisms, and (hopefully) putting it all back together in working order.
If I had an old lens where I could see fogging on the internal glass surfaces, where the focus mechanism was uneven, too stiff or too loose, or ditto the aperture mechanism, I'd consider having it CLA'd. I have no hard data on what it'd cost for a lens though; for what it is worth I did pay a couple hundred Euro for CLA on a Leica M3 camera body recently.