Membrillo literally translates to "quince", a fruit similar to a pear. However, in Spain, it's a colloquial term used to call someone a dunce or an idiot!
In this powerful rock anthem, the line "No seas membrillo y permite pasar" (Don't be a dunce and let me pass) is a direct and forceful command. The singer is using this unique piece of slang to tell someone to stop being stubbornly foolish and get out of his way.
This song is a full-on rock explosion, and it's basically one long, furious message to someone who needs to pick a side. The singer is completely fed up with a person who is stuck in the middle, trying to have it all and making life impossible for everyone else. The main line says it all: "Entre dos tierras estás / Y no dejas aire que respirar" (You are between two lands / And you don't leave any air to breathe). It's a powerful way of saying this person's indecision is suffocating.
The lyrics are incredibly direct. The singer accuses this person of being willing to "vender" (sell yourself) for power and of talking too much without thinking. The message is clear: make a choice, but also, just go away. The singer washes their hands of the whole situation with the line "Déjame / Que yo no tengo la culpa de verte caer" (Leave me / It's not my fault to see you fall). It's a cold, blunt dismissal of someone who has made their own mess.