Perdida literally means "lost (female) one". However, in the song's title phrase "¿Qué onda perdida?", it's a very common and informal Mexican slang way of saying, "What's up, stranger?" or "Long time no see!"
It's used to address someone you haven't seen or heard from in a while, as if they've been 'lost'. In this context, the singer uses it as a bold and familiar way to reconnect with his ex, mixing a bit of playful accusation with underlying affection.
This song is one big, bold move from an ex who is absolutely sure he's not forgotten. The singer calls up his former love, cheekily asking, "¿Que onda perdida?" [What's up, lost one?]. He's noticed she hasn't blocked him on WhatsApp and takes this tiny detail as a huge sign, thinking, "podemos comenzar de nuevo" [we can start over again]. He's not just any ex; he confidently calls himself "el cabrón que más te ha querido" [the bastard who has loved you the most] and admits he hasn't even tried to forget her.
He doesn't just send a text, though. He tells her to get ready because he's planning a massive romantic gesture: "te me alistas porque hoy te llevo la banda" [get ready because today I'm bringing you the band]. He's going to show up at her house and refuse to leave until she comes out and tells him to his face that she doesn't love him, doesn't miss him, and that "alguien mejor, está ocupando tu cama" [someone better is in your bed]. It's a dramatic, all-or-nothing challenge to see if she really has moved on.