The following contains spoilers for the Attack on Titan manga, not yet covered in the anime.
It's finally happening, the final chapters of Isayama's incredible story, Attack on Titan. Known for its powerhouse anthems of an anime opening, in the Attack on Titan Final Season the Final Chapter Special 1, fans were startled that MAPPA decided to forego an opening completely. That was until we reached the episode and saw the credits. Almost every song associated with the anime has been a phenomenal hit with the fans. From the beginning with Linked Horizons' "Guren no Yumiya" and "Shinzou wo Sasageyo" to Hiroyuki Sawano's "Call of Silence" and Ai Higuchi's "Akuma no Ko", every single opening and ending has been packed with references and emotional intent. SiM is the band bringing us home in the final season with not only their iconic opening "Rumbling" but with the ending song "Under the Tree" as well. Just the title itself if loaded with allusions to the manga's conclusion, to the point where the final chapter is called "Toward that Tree on That Hill".
As the story draws to a close, "Under the Tree" heralds the conclusion specifically between Eren and Mikasa. Episode one, "To You, in 2000 Years: The Fall of Shinganshina Part 1", starts off with Eren waking up underneath a tree in his hometown with Mikasa nearby, watching over him. The Final Chapter also starts off at the same location, a spot of childhood innocence and a blissful moment of peace where these two characters could relax in each other's company. It is stated that this tree is one of Eren's favorite napping spots, and Mikasa and Armin frequently visited this location with him. In SiM's "Rumbling" opening, the three of them are actually shown to run towards this same tree where multiple different versions of Eren from throughout the show stand waiting.
SiM "Under the Tree" Cover
The song is told from Mikasa's point of view, clearly referencing her red scarf in the lyrics, so with the songs opening lines of questions, she is asking why Eren went somewhere she couldn't follow. Throughout all of season four, no other character has quiet been so thrown off as Mikasa Ackerman, the person who knew Eren better than his own parents. His decision to start the Rumbling even though she has always known him to care about the lives of strangers from personal experience, and his dismissal of her from his side and his heart. Eren told Mikasa he has always hated her, and while it was the worst thing she's ever heard, she couldn't make sense of it either. Eren has always gone out of his way for her, even promising to wrap her scarf around her forever.
Her desperation to understand him in season four and to process whether or not he's gone pass the point of no return is echoed perfecting in the series of questions the song starts out with. In one specific lyric, Mikasa seems to be begging "don't make a mess of memories/ just let me heal your scars". Eren had previously tried to convince Mikasa that she is a slave to her Ackerman blood, and that's the only reason she feels anything at all towards him, completely devaluing her love. Even when so thoroughly hurt, she is still trying to protect Eren.
Eren's Tree
The song then states anime specifics in their lyric "the wall, the owl, and forgotten wharf". While the first one is pretty self-explanatory for Attack on Titan, the Owl is a reference to Eren's namesake, Eren Kruger, an Eldian spy working to save his people. As the previous Attack Titan, Kruger gave his titan to Grisha Jaeger, explaining that he has done terrible things to his own people while undercover as a Marleyan, all in the name to save them. Eren has used this man's last name as an alias while in Marley, showing how he reflects the same methods. The forgotten wharf is the only other man-made structure on Paradis that is not from the Walls. It is the location of the Marleyan death sentence for Eldians, where they are pitched from the top of a concrete structure to turn into titans.
In the ending itself, the audience can only see a person walking on a dirt path, similar to the "Akuma no Ko" ending where Eren is walking alone through a field of flowers, until the chorus hits multiple stills of birds flying with the word "why" being repeated over and over. Attack on Titan has always had bird imagery involved with ideas of freedom and entrapment. The Walls themselves are often referred to as a bird cage. In the end of the manga, Isayama used the image of a bird pulling on Mikasa's scarf, wrapping it around her one last time, and as the lyrics states, Mikasa waited for Eren there.
Mikasa, Waiting Under the Tree
This is the same tree is where Mikasa buries the only part of Eren she has, the head she severed from his shoulders. She visits that grave constantly, long after Eren has died. She introduces her child to him at that grave, and she still visits even when she's an old woman.
As the cycle of violence rotates once again, and time has passed and all of the characters we know have died, war ravages the land until the whole city is wiped away. A forest grows where Eren's tree stands tall and proud, and a child is seen stumbling though the forest. Eren's tree is large enough that there is an opening right at the base of the trunk, exactly the same way the Founder Ymir came in contact with the power of the Titans the first time. The story starts anew, under the tree.