入れる is a common verb meaning to put in, insert, or include. It implies moving something from the outside to the inside of a container or space. It is also used in the context of making tea or coffee (お茶を 入れる).
That is how Fuguro works. It brings 入れる 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.