Calan comes from the verb calar, which means to pierce, penetrate, or soak through. While it can refer to physical penetration, in this song, it's used metaphorically to describe how memories deeply affect someone.
Christian Nodal sings, "Me calan tus recuerdos" (Your memories pierce me deeply). This powerful imagery conveys the intense emotional pain and lasting impact of past memories, making it a more evocative choice than a common verb like "doler" (to hurt).
Nodal builds this one out of two addresses. There is aquí (here), his side, and allá (there), hers, and the whole song is the distance between them. On his side: "Aquí, donde te extraño / Donde no pasa el tiempo" (Here, where I miss you / Where time doesn't pass). On hers: "Tú allá, donde te vale / Y donde nuestro amor ya te da igual" (You over there, where you can't be bothered / And where our love means nothing to you now). He never accuses her of anything worse than indifference, which somehow lands harder than an accusation would.
The opening does the arithmetic. "Cuántas noches pa' olvidarte van / Gastadas y no puedo" (How many nights have gone by trying to forget you / Spent, and I can't), and then the line that gives the whole thing its ache: "Si te extraño hasta los huesos" (I miss you down to the bones). The cruellest detail arrives later, when he admits she enjoys the situation: "Te gusta que se sepa / Que sigo siendo tuyo" (You like people to know / That I'm still yours). He says it anyway, and the mariachi swells behind him as if the arrangement had taken her side.