大きな is a pre-noun adjectival (れんたいし) meaning big or large. It can ONLY be used directly before a noun. Compared to the い-adjective 大きい, it often carries a more subjective, emotive, or abstract nuance, and is common in storytelling.
That is how Fuguro works. It brings 大きな 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.