入る is an intransitive ごだん (Type I) verb meaning to enter or to go in. It is used when the subject moves into a physical space (like a room) or joins an organization (like a university). It is distinct from the transitive verb 入れる (to put in).
That is how Fuguro works. It brings 入る 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.