Carrerilla literally means a "running start" or "short run." However, in the context of "aprendo la lección de carrerilla," it takes on a figurative meaning: to learn something "by rote" or "by heart" without deep understanding.
It's an interesting word because it highlights the idea of superficial learning, which contrasts with the deeper reflections in the song about life's lessons and discoveries. This dual meaning makes it both unique and thought-provoking.
Despistaos build this one around a game of musical chairs that never quite ends. 'Jugábamos al juego de la silla / Y me quedé colgado' (We used to play musical chairs / and I was left hanging) sets up the whole song: two people who kept circling each other until only one seat was left, and he's the one still standing.
The rest of the lyrics keep finding new images for that same stuck feeling. 'Se funden las bombillas' (The light bulbs are burning out) repeats twice, like something breaking down that nobody's fixing. He's swum most of the way to shore but 'Pero me va a costar llegar a nado' (but it will cost me to get there swimming), and by the bridge he's worn his shoes down to nothing, 'un ciego entre los tuertos' (a blind man among the one-eyed), pretending worse than everyone else around him. The hook argues against all of that: 'Que dos más dos son uno' (that two plus two make one), the two of them adding up to something whole instead of splitting into halves. It keeps circling back to that line, like it's the one thing he still believes.