Most homebrewers start out using a single-stage fermentation, often in a covered plastic bucket. In addition to homebrewers, many European and even some American craft brewers follow this practice. It is certainly one example of secondary fermentation. But for homebrewers, the phrase has a different meaning. Here, it more often refers to the final phase of fermentation before bottling.
Primary fermentation is marked by the vigorous release of carbon dioxide.
The airlock on top of a typical homebrew fermenter will bubble away at a rapid rate during this phase. As this vigorous activity winds down and the bubble count decreases, the primary fermentation is drawing to a close. The entire primary fermentation phase usually only takes three to five days in a normal ale fermentation. Despite this, the yeast still has important work to do in the beer. During this latter phase of fermentation, the majority of the yeast in the beer also settles to the bottom of the fermenter, leaving the beer clear and bright.