Diluyendo comes from the verb diluir, meaning "to dilute" or "to dissolve". It's a less common, more evocative word than simply saying something "disappeared" or "ended".
In the song, Sebastián Yatra uses it to describe how his sadness over a past relationship "se fue diluyendo" (faded away or dissolved) over time. This imagery of sadness slowly dissolving like a substance in liquid is quite poetic and memorable, making it an interesting word choice for learners.
Sebastián Yatra opens with a confession: "Empezamos todo demasiado tarde" (We started everything too late). This is a breakup song that refuses to turn bitter. He admits it fell apart, but says what's left is "La tristeza por ti" (the sadness over you) fading out, while the tenderness stays. Three lines in a row carry that: "Todavía te pienso" (I still think of you), "Todavía te siento" (I still feel you), "Todavía te quiero" (I still love you).
The title line is the promise the whole song is built around: "Aquí estaré" (I'll be here), offered to someone he hopes will find "Que te acompañe" (someone to be with you), and if she doesn't, he will. He owns his side of the breakup too, singing "No estás loca, la vida es medio loca" (You're not crazy, life is a bit crazy), and "Me matas, me matas cada vez cuándo" (You kill me, you kill me every time) admits it still stings without turning that into blame. It reads as a quiet, stubborn kind of loyalty, staying present for someone even after love has changed shape.