Introvabile literally translates to "unfindable" or "impossible to find." It's a striking and somewhat dramatic word that isn't used in everyday conversation, making it memorable.
In the context of the song, the artist uses it to describe a person who is elusive and hard to pin down, both physically and emotionally. He sings, "E poi dico che tu sei introvabile" (And then I say that you are unfindable), highlighting the frustration and longing associated with someone who is constantly out of reach. This word perfectly encapsulates the central theme of a difficult, on-again, off-again relationship.
Bresh spends this whole song running away and then complaining that nobody is there. About ninety seconds in he admits it outright: "Ma quanto è ipocrita, e capisco quando dico che sparisco / E poi dico che tu sei introvabile" (But how hypocritical it is, and I understand when I say that I disappear, and then I say that you're the one who can't be found). He knows exactly what he is doing. He does it anyway.
The small lines are the good ones. He wants to stop "come un treno quando tiri forte il freno" (like a train when you pull hard on the brake), which is a lot of screeching and not much stopping. He calls himself "un cronico ritardatario" (a chronically late man) about a woman who actually waited. Then he drops the arguing and finds the image that does the work: "E io sono come un biglietto che hai lasciato dentro un libro / Che cadrà sul pavimento quando cambierai l'appartamento" (And I'm like a note you left inside a book that will fall on the floor when you move apartment). Someone hoping to be found by accident, years later, by a person who has already packed.