Short and plain, "Back together" is about the aftermath of heartbreak. The speaker says someone else was responsible for breaking them, and the first two lines state it flatly, without anger or elaboration. "You were the one / That broke me" reads like a simple fact the speaker has had a long time to accept.
The second half shifts from blame to something sadder. The speaker is "still waiting" for "someone" to repair them. That word is vague, and it's worth noticing: it may not be the person who did the damage, just anyone willing to help. The speaker seems to see themselves as broken, like an object that can't fix itself, and passive, hoping rescue will arrive.
The mood is quiet, tired, and vulnerable. There's no dramatic outburst, just a lingering ache and a hint of loneliness. The short lines and the gaps between them leave space that feels like the waiting itself.