Lagunas literally translates to "lagoons", but it has a fascinating double meaning. In Spanish, the phrase "tener lagunas mentales" means to have gaps in your memory, a moment of mental confusion.
In the song's central line, "Nado entre lagunas de mi mente" (I swim in the lagoons/gaps of my mind), the singer creates a beautiful metaphor. He's poetically describing the feeling of being lost and adrift in his own thoughts and memories of a past love.
This song is all about the pain of staying away from someone you still love. The singer has decided that distance is the only answer, saying, "De lejos es mejor / No por mí, esto es por los dos" [It's better from a distance / Not for me, this is for both of us]. He's trying to be strong, but his own mind is working against him.
He describes being lost in his thoughts as "Nado entre lagunas de mi mente" [I swim in the lagoons of my mind], which is such a cool way to say he's drowning in memories. These mental lagoons are filled with daydreams of what it would feel like "Volver a besar tu frente" [To kiss your forehead again]. He even imagines a different reality where things worked out, wondering, "Tal vez en otra galaxia sí fue diferente" [Maybe in another galaxy it was different]. It's a sad but beautiful look at a love that can only exist in his imagination.