This part of the brewing process is the slowest and requires the most patience. This is where you, the brewer, do nothing but there is something happening to the beer, and it’s being done by yeast.
Yeast are a particular strain of fungus that have evolved to be particularly useful to the Brewer. They are perfectly suited to the environment created by your beer kit and have all the food (sugar) and nutrients they need to multiply and reproduce. This is vitally important because without yeast we wouldn’t have any alcohol at all. They also provide a large proportion of flavour to the beer so making sure they are happy is important so they perform as well as possible.
Once you have pitched yeast into your fermenting bin containing the sweet malty liquid they begin reproducing and multiplying. As they multiply they consume the sugars available and the by-product of this is alcohol. The more sugar available to the yeast the higher the alcohol content of the beer (within reason). The amount of sugar available to the yeast is how brewers can control the alcoholic content of their beers.
As I said you have to do nothing during fermentation, except that is, to make sure the fermenting beer is in an ideal environment for the yeast this means keeping the fermenter in a spot where the temperature is consistent and ideally around 17°C – 22°C. If the beer gets too hot the yeast will gradually die and leave you with unfermented beer. Too cold and they go into a sort of hibernation and again won’t ferment the beer probably. Keep them around room temperature and you will have a great beer to enjoy.