本in Japanese primarily means book or volume. As a kanji character, it also represents origin, root, true, or main. Additionally, hon serves as the Japanese counter for long, cylindrical objects (like pens, bottles, or trees).
That is how Fuguro works. It brings 本 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.