Hi Mr Fei,
It is a bearing made out of a higher quality steel (stainless steel) then other bearings. Many bearings of lesser metals will not hold up over time when saltwater fishing. Stainless steel bearings are more resistant to corrosion and moisture. Also stainless steel bearings have superior hardness. They grade ball bearings and the better fishing reels have a higher quality stainless steel ball bearing in them.
Just to name a few Shimano calls their bearings ARB bearings, standing for Anti-Rust Bearings. Daiwa calls their bearings CRBB bearings, standing for Corrosion Resistant Ball Bearings. Even the terminal tackle like ball bearing swivels (which are better than barrel swivels) can have stainless steel ball bearings. They will cost more, but last longer and be much smoother.
When purchasing a fishing reel, I highly recommend that the reel have quality stainless steel bearings inside. It is well worth the money!!