Falena translates to "moth". It's a less common word in everyday Italian, making it stand out in the lyrics.
In the song, it's used in the line "La ragazza ingenua come una falena alla luce va di una fiamma tremula" (The naive girl, like a moth, goes to the light of a trembling flame). This metaphor beautifully portrays innocence and vulnerability, drawn to something potentially dangerous, much like a moth to a flame.
"Il Mostro" (The Monster) is Gianluca Grignani's portrait of addiction, sung as if the drug itself is speaking. The "monster" comes to take a naive girl away, drawn to it "like a moth to a flickering flame."
The chorus is the monster talking directly to her: it promises to lift her up, then knocks her right back down. That's the trap the whole song describes: something that looks like relief but is actually the sickness itself.
The middle section follows what happens once it takes hold, chasing her, carrying her off, and warning her not to confuse real feeling with what it offers. By the end she's just watching it happen, repeating "look, look, I'm the monster" as a warning she can't outrun.