行く is a fundamental ごだん (Type I) verb meaning to go. It describes movement away from the speaker's current location toward a destination. Note that its て-form and past-tense forms (行って, 行った) are irregular.
That is how Fuguro works. It brings 行く 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.