Secondary fermentation: when it helps and when not
Most beers do not need a secondary vessel. Knowing the few that do will save you work and reduce the chance of oxidation or infection.
What it is
In a secondary fermentation the beer is moved off the yeast into a second, sanitised vessel after the main fermentation, for further conditioning.
When it helps
A second vessel makes sense for beers that need weeks or months on something other than yeast: fruit, oak, spices, or long dry hopping. It also frees up your primary for the next brew.
When to skip it
For a normal-strength ale, leaving the beer on the yeast for two or three weeks gives a cleaner flavour than moving it. Every transfer risks oxygen and infection, and modern yeast does not cause off-flavours from sitting on the cake.
If you do rack
Sanitise the second vessel, siphon gently from the side to avoid splashing, and fill it as full as you can so there is little air above the beer. Keep an airlock on until you are sure no more fermentation is happening.