Warning: Spoilers for Stranger Things season 3.
Here's Hopper's full message to Eleven from Stranger Things season 3. Stranger Things season 3 ended with an explosive, climactic, and heartbreaking finale. As Eleven and her friends - Mike, Max, Lucas, Jonathan, and Nancy - all gathered to fight the Mind Flayer, Sheriff Jim Hopper and Joyce, along with Russian translator Murray, set off to an underground Russian bunker to close the portal to the Upside Down. While their mission was successful, it came at a price, and that was the life of Jim Hopper.
Hopper may still come back, of course. We don't know for sure that he's actually dead, and given that this is a sci-fi show, there are plenty of ways to potentially bring him back. But for now, at the end of Stranger Things season 3, Eleven, Joyce, and the kids of Hawkins are all in mourning. The final scenes of season 3 showed Joyce selling up and moving on with Jonathan, Will, and El, who moved in with the Byers after Hopper died.
As Joyce folded Hopper's Sheriff's uniform, she found the speech that Hopper had written to El, with Joyce's help. While he was meant to learn it and recite it to her, he actually bottled it and ended up threatening Mike instead. However, Joyce handed the speech over to Eleven, who read it, and it turned out to be pretty sound life advice for anyone trying to navigate or parent someone through their teenage years. The scene played out with Hopper's narration as the Byers family plus El left town. Here is Hopper's speech in full:
"Feelings. Jesus. The truth is, for so long, I'd forgotten what those even were. I'd been stuck in one place; in a cave, you might say. A deep, dark cave. And then I left some Eggos out in the woods, and you came into my life. For the first time in a long time, I started to feel things again. I started to feel happy. Lately, I guess I've been feeling distant from you. Like you're pulling away from me, or something. I miss playing board games every night, making Triple Decker Eggo Extravaganzas at sunrise, watching Westerns together before we doze off.
"But I know you're getting older - growing, changing. I guess, if I'm being really honest, that's what scares me. I don't want things to change. So I think that's maybe why I came in here, to stop that change. To turn back the clock. To make things go back to how they were. But I know that's naive; that's just now how life works. It's moving, always moving, whether you like it or not.
"And yeah, sometimes that's painful. Sometimes it's sad. And sometimes, it's surprising, happy. So, you know what? Keep on growing up, kid. Don't let me stop you. Make mistakes. Learn from them. When life hurts you, because it will, remember the hurt. The hurt is good. It means you're out of that cave. But please - if you don't mind, for the sake of your poor, old dad - keep the door open three inches."
While Hopper and Eleven didn't interact much in Stranger Things season 1, he's the person who accepted her into his life in season 2. He ultimately raised El as his own daughter, which became official at the end of Stranger Things season 2 thanks to former Hawkins Lab scientist Dr. Sam Owns (Paul Reiser). And throughout their time together, it was clear that, despite their arguing and disagreement on often El should see Mike, it seems that they truly loved each other, as evident by Hopper's speech and El's emotional response to reading it.
Sure, Hopper had some help in the beginning with writing the speech, but in the end, he spoke from his heart. And so, while Hopper wasn't there to actually give this speech to El himself, they did have a heart-to-heart, just like Joyce suggested. Whether or not Hopper does return for Stranger Things season 4 remains to be seen, but regardless, Eleven will undoubtedly heed his advice as she grows into a teenager.