Qué onda is a classic and very common Mexican slang phrase for "What's up?". It's an informal greeting you'll hear all the time.
Perdida literally means "lost one", but here it's used colloquially to address someone you haven't seen or heard from in a while. The full phrase, "Qué onda perdida", is a bold and catchy way an ex-boyfriend starts a conversation, immediately setting the confident and slightly mischievous tone of the song.
This song is a bold, maybe even a little crazy, attempt to win back an ex. The singer calls her up, saying, "Qué onda perdida / Soy tu ex" (What's up, lost one / I'm your ex). He's noticed she hasn't blocked him on WhatsApp and takes that as a sign he still has a chance. He's so sure of their past connection that he declares, "Uno siempre vuelve a donde lo quisieron" (One always returns to where they were loved).
He doesn't just send a text, though. He goes all out. He warns her, "Te me alistas porque hoy te llevo la banda" (Get ready because today I'm bringing you the band). He's literally showing up at her house with a full musical group and refuses to leave until she comes out. He's demanding a final answer, challenging her to tell him to his face "Que ya no me quieres... / Y que alguien mejor está ocupando tu cama" (That you don't love me anymore... / And that someone better is occupying your bed). It's a huge, dramatic gesture from the self-proclaimed "cabrón que más te ha querido" (the bastard who has loved you the most).