なる is a ごだん (Type I) verb meaning to become. It indicates a change from one state to another. When used with a noun or a な-adjective, it is preceded by the particle に. With i-adjectives, the final ~i changes to ~く (e.g., 寒くなる - to become cold).
That is how Fuguro works. It brings なる 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.