Clavos literally translates to "nails," like the metal fasteners. It's an unexpected and intriguing word to find as the central theme of a song.
In the context of the lyrics, "somos clavos" (we are nails) and "tú te estás clavando en mí, yo me estoy clavando en ti" (you are nailing yourself into me, I am nailing myself into you) is a powerful metaphor. It suggests a painful, inescapable, and perhaps destructive entanglement, where two people are deeply, almost painfully, stuck together, much like nails driven into something.
"Clavos" (Nails) is about two people who keep coming back together even though they know it will hurt again.
Both of them carry old wounds from past relationships, and instead of promising anything real, they admit they are drawn to each other out of habit and shared pain rather than love. The chorus plays on the Spanish verb clavarse (to nail oneself into something, or to fall hard for someone): "you're nailing yourself into me, I'm nailing myself into you" - a picture of two damaged people piercing into each other again, aware it will leave another scar.
It's a song about repeating a pattern you can already see coming, because the loneliness of stopping feels worse than the pain of staying.