In Spanish, this phrase is a classic example of an impersonal sentence. We don’t always know who "they" are; the focus is entirely on the action and the mystery of what lies on the other side of the threshold. The Mystery of the Unknown
(Approach with caution and skepticism).
The phrase also resonates through music and literature. Fans of Camila Cabello might recognize the title from discussions around her hit "My Oh My," which features an interpolation of the 1992 Peruvian reggae track "Llaman a la puerta" by Tierra Sur. In these contexts, the "knock" often symbolizes a forbidden romance or a temptation that is impossible to ignore. Llaman a la puerta