Lager (which is short for Lager beer) is a kind of beer. It is made from a kind of yeast. Fermentation will be a slow process. It usually has a clear color. That color comes from the beer being stored for some time. During that time, the yeast can settle at the bottom of the barrel. The yeast is removed before the beer is bottled or served.
Lagers are usually drunk while they are cold.