Atemporal translates to "timeless" or "ageless". It's a sophisticated and evocative word that stands out from everyday vocabulary, making it memorable.
In the song, Christian Nodal uses it to describe a love that transcends the passage of time, singing, "Pa' que entiendas que lo nuestro va a ser algo atemporal" (So you understand that what we have will be something timeless). This word beautifully captures the enduring and eternal nature of the love he expresses.
Nodal spends the first half of this song making promises and the second half talking himself out of them. The pitch is speed, or the lack of it: "Te voy a amar despacio cuando el mundo vaya aprisa" (I'm going to love you slowly when the world goes fast). Then comes the turn, and it's the most honest moment here. "Quiero prometerte tantas cosas / Pero quien promete dicen que jamás lo va a cumplir / Por eso mejor te traje rosas" (I want to promise you so many things, but they say whoever makes promises never keeps them, so I brought you roses instead). He drops the vow mid-song and hands over flowers, then lets the flowers do the talking: "Pa' que ellas te platiquen lo que yo siento por ti" (so they can tell you what I feel for you).
The waltz of the title shows up once per chorus, and it is a small plan rather than a forever: "Y un día bajo la lluvia hemos de bailar un vals" (and one day under the rain we'll dance a waltz). One day, one dance. That modesty is what keeps the song from tipping into syrup, and it is why the line about hard times sounds so matter-of-fact: "Porque esta vida como tiene malo, tiene bueno" (because just as life has its bad, it also has its good).