Coartada is a Spanish word for "alibi". It's a legal term you wouldn't expect to hear in a pop song, which makes it a unique and intriguing word to learn.
In the lyrics, Íñigo Quintero sings, "No tengo coartada, borré las pisadas" (I have no alibi, I erased the footprints). He uses this word metaphorically to express a feeling of being caught in his emotions with no excuse or way to escape the past, adding a layer of drama and mystery to the song.
This song feels like you're trapped inside your own head after a tough breakup. Íñigo Quintero starts by saying he's in a battle with his own thoughts, Los pensamientos a contracorriente (thoughts going against the current), and feels like he hasn't seen the sun in ages. He's dealing with someone he describes as inmortal (immortal) and sobrenatural (supernatural), a person who nunca vas de cara, no te quitas el disfraz (never shows your true face, you don't take off your disguise). It's this mysterious, maybe even toxic, person who has left him completely stuck.
The core of the song is this feeling of being paralyzed. He uses the incredible image of having a sword stuck in him: Con la espada clavada, no me dices nada (With the sword stuck in me, you say nothing). Because of this pain, he feels like he has Sin tiempo para bailar (No time to dance), no time or energy to enjoy life. But it's not all doom and gloom! By the end, there's a shift. He decides he's going to become a mejor versión (a better version) of himself, one who has killed the sorrow. The kisses are now just a memory from a invierno / Que no volverá jamás (winter / that will never come back), and this song is his final word on the matter.