The fermenter is sealed with the lid to protect the wort from air born contaminants.
An air lock is filled partially with clean water and inserted in the lid using a rubber grommet to provide the seal.
The fermenter is stored in a cool dark place; left to ferment. It should be checked periodically to make sure that the air lock is still releasing the fermentation gasses. Occasionally, air locks can be clogged with brewing sediment that rides on top of the foam produced by the initial vigorous fermentation. If there is no activity within 24 hours, re-hydrate a fresh new packet of yeast and add it to the wort because the first packet used may be dead.

This stage of fermentation lasts from 3 to 5 days, sometimes less. When fermentation has slowed, the beer can be transferred off the sediment left from primary fermentation into the secondary fermenter.