Reading at a 6th grade level is reading for plot. Just like, what happened? Who was there? More advanced things like subtext, metaphor, and unreliable narrators come later.
What does it mean practically? Bad things. If you haven't read 1984, give it a go and think about why the authoritarian state benefitted from a diminished language.
Aside from the seminal "Shake It Off" (which itself has been analysed by scholars to death), 1984 shows Taylor Swift's writing prowess with bars such as
"Now we got bad blood/ You know it used to be mad love."
I don't know how that test compares to grade level. It seems like a test to determine one's CEFR level, 20 correct answers out 20 gave me C1. It doesn't really say what it's based on, but it does encourage you to buy their product.
At first, she thought it was difficult, but when she finished school, she could already speak quite fluently and understand almost everything she heard or read.