Hueco literally means "hole" or "gap." While it can refer to a physical space, it's often used metaphorically in Spanish to describe an empty space or a void.
In this song, Silvana Estrada uses it beautifully in the line, "Si me hubieras hecho un hueco en tu corazón herido" (If you had made a space for me in your wounded heart). Here, hueco signifies a place or opportunity for her in the other person's emotional life, making it a poignant and evocative word choice.
The title is a lie the singer keeps telling herself. Cada día te extraño menos (Every day I miss you less) opens the song, and every chorus lands on it too. She just never gets through it without an amendment: Aunque siempre me hagas falta (Although I always miss you), and by the last chorus only Aunque me duela (Even though it hurts me). The song lives in that gap between the claim and the correction, and the correction always gets the last word.
What keeps it from tipping into self-pity is how concrete Silvana Estrada stays. Memory is a dance neither of them can stop: El recuerdo es una danza, ay / Que nos tiene dando vueltas (The memory is a dance, oh / that has us spinning). Grief is weather she hauls around behind her: El viento arrastra sus nubes / Así arrastro yo mis penas (The wind carries its clouds / This is how I drag my sorrows). When a tear actually lands she talks herself out of it mid-line: Mejor pensar que es la lluvia (Better to think it's rain). The sharpest moment is the quietest one, an accusation folded into a conditional: Si me hubieras hecho un hueco / En tu corazón herido (If you had made room for me / In your wounded heart). The heart she wanted a place in was already wounded, and the room was never made.