酒refers to alcohol or specifically Japanese rice wine (にほんしゅ). In general conversation, it is the broad term for any alcoholic beverage. The word may originate from さかえ (to prosper) due to its use in celebrations.
That is how Fuguro works. It brings 酒 back for review right before you forget it, each time a little later, until it sticks for good. You hear it spoken aloud, and you meet it again in stories you can already read. The first five levels are free.