想像refers to the mental act of creating an image or idea of something that isn't currently present or hasn't happened. It is often used in the phrase 想像がつかない (*そうぞう が つかない*), meaning I can't even imagine it.
That is how Fuguro works. It brings 想像 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.