Cornisa literally means "ledge" or "cornice", referring to the decorative molding at the top of a building. It's a specific architectural term you don't often find in song lyrics.
In the song, Gustavo Cerati sings "Estamos al borde de la cornisa, casi a punto de caer" (We're on the edge of the ledge, almost about to fall). This creates a powerful, cinematic image of tension and anticipation, putting the characters in a risky, high-stakes situation that perfectly captures the thrill and suspense of the song's voyeuristic theme.
This song is all about watching someone from a distance. The singer takes on the role of a spy, saying, "Soy un espía, un espectador" (I am a spy, a spectator), as he watches someone through a Venetian blind, the "persiana americana" of the title. This isn't a creepy, one-sided thing, though. It's a game between two people who enjoy the tension.
The whole scene is built on anticipation. The singer prefers the person he's watching to be "inalcanzable" (unreachable) and "casi intocable" (almost untouchable). He knows the other person is enjoying the game too, singing, "Sé que te excita pensar hasta donde llegaré" (I know it excites you to think how far I'll go). It's about the thrill of the moment just before something happens, a feeling he calls "una condena agradable" (a pleasant sentence) and compares to being "al borde de la cornisa / Casi a punto de caer" (on the edge of the ledge / Almost about to fall).