Racking is transferring beer, cider, or mead from one vessel to another while deliberately leaving sediment (spent yeast, trub, or fruit pulp) behind — traditionally done with a racking cane and siphon tube, positioned to draw liquid from above the settled solids rather than disturbing them.
Every transfer is a real oxidation and infection risk
Racking is also the moment most homebrew batches face their biggest post-fermentation oxidation and contamination risk — an open, splashing transfer introduces both oxygen and airborne contaminants directly into beer that no longer has active fermentation CO2 protecting it. A gentle, low-splash technique (submerging the racking cane's outlet below the receiving vessel's liquid surface as soon as there's enough liquid to do so) meaningfully reduces both risks compared to a fast, careless transfer.
Auto-siphons and gravity siphons both work, differently
A gravity siphon needs the source vessel positioned higher than the destination and typically needs starting by mouth-suction or a priming method; an auto-siphon starts itself with a pump action and doesn't need height difference the same way, at the cost of being another piece of equipment to sanitize. Either works fine for standard racking — the choice mostly comes down to convenience and existing equipment rather than one method producing a meaningfully better result.