Eleven thinks she understands what he means. She might have been to the Upside Down, she might have fought the demogorgon, and demodogs, and Papa and his orderlies... but even despite that, she's never seen the sorts of things Hopper has. She can't imagine what they could be, how bad they could be.

He's trying to protect her.

"But maybe I can stop it from happening," she reasons. "So then, nobody has to see it."