Feedback Says My Writing Is "Dense"--What Does This Mean and What Can I Do About It?

A student in my online classes is writing a futuristic thriller about memory loss.  I've enjoyed reading her chapters in class and so have her classmates.  But recently she emailed me about some feedback she'd received that she didn't understand.  She said she couldn't find much information about online, so she was hoping I could help her with what to d…