Secondary fermentation is a period of aging that occurs after siphoning or transferring your fermented beer to a secondary vessel after primary fermentation is complete. Contrary to its name there is actually very little to no yeast activity in a secondary fermentation, because of this it is also referred to as the conditioning phase. Transferring your beer to secondary will allow the beer’s flavors and aromas to mellow and let yeast to drop out of solution, producing a clearer finished product.
For many beers with an original gravity of 1.040 or lower, or beers that are usually served cloudy, this step is usually not necessary. It is almost always used for higher gravity and highly hopped beers that need time to age before reaching their peak flavor and aroma. You will also need to employ a secondary fermentation when adding dry hops or oak chips.
The duration of a secondary fermentation or conditioning phase can vary from as little as a week to over 6 months. Actual time will vary and you should let your taste buds and nose determine when a beer is ready for bottling. During extended secondaries, you should make sure your airlock does not dry out. For this reason, a bubbler airlock is best for secondary fermentations.
A growing number of brewers believe that a secondary fermentation is unnecessary and an extended primary of 3-4 weeks will produce the same results. There are not extensive studies on this matter to prove these claims but it remains a viable alternative to transferring to secondary before bottling.