Cántaro is a traditional pitcher or earthenware water jar, a more poetic and specific word than the everyday jarra. In the song, the emptying soul is compared to a cántaro en la nube, creating a striking image of love gradually vanishing like water disappearing into a cloud.
Natalia Lafourcade builds this whole song out of one stubborn sentence: "El amor acaba" (Love ends). Every verse is a new reason, and every reason starts with "Porque" (Because), so the song plays like someone listing evidence in a case they already know they have won.
The reasons are tiny pictures. The soul empties out "como el cántaro en la nube" (like the water jug in the cloud), and a caress slips away "como sombra" (like a shadow). My favourite is "se vuelven cadenas / lo que fueron cintas blancas" (what were once white ribbons turn into chains). Think wedding ribbons becoming shackles. That one stings. She even blames routine: "llega a ser rutina / la caricia más divina" (even the most divine caress becomes routine).
Then it gets philosophical. "Somos como ríos / cada instante, nueva el agua" (we are like rivers, new water every instant) says we are never the same person twice, so how could love stay the same? The last lines are the bluntest: "Porque nada es para siempre / y hasta la belleza cansa" (Because nothing is forever, and even beauty gets tiring). Quick grammar tip: "acaba" comes from acabar (to end, to finish), a verb you will hear constantly in Spanish.