Aguacero literally means "downpour" or "heavy rain shower". It's a much more intense and evocative word than the common lluvia (rain).
In this song, Ricky Martin uses it as a powerful metaphor. He sings, "Tu recuerdo sigue aquí, como un aguacero" (Your memory is still here, like a downpour), to describe how the memory of a past love suddenly and intensely overwhelms him. It perfectly captures that feeling of being completely drenched in nostalgia.
This song is a fantastic duet about a memory that just won't fade after a breakup. Ricky Martin kicks it off by describing this memory as a powerful storm: "Tu recuerdo sigue aquí / Como un aguacero" [Your memory is still here / Like a downpour]. But it's not just rain; it's a storm that "Quema y moja por igual" [Burns and soaks at the same time]. He's completely stuck, wondering if this constant reminder of his ex "me hace bien o me hace mal" [is good for me or bad for me]. He admits he was the one who left, but the memory stayed behind, sometimes feeling like a comfort and other times like it wants to kill him.
Then, Chambao gives her side of the story. She flips the script, singing "Que tú te fuiste, eso es pasado" [That you left, that's in the past], and agrees she needs to forget him. But she's not making it easy on herself! In a wonderfully dramatic move, she admits, "yo le puse una velita a todos mis santos... para que pienses mucho en mí" [I lit a candle to all my saints... so you'll think a lot about me]. They both know the memory is "antídoto y veneno al corazón" [antidote and poison for the heart], a feeling that's both healing and destructive.