Murailles translates to "walls", but it's a more evocative and less common word than the everyday "murs". It often refers to ancient or fortified walls, carrying a sense of history and permanence.
In the song, the narrator is waiting and listening to "parler les murailles" (the walls speak). This personifies the walls, suggesting they hold countless stories, emotions, and the graffiti of human experience, making the word particularly poignant in this context.
"Défense d'afficher" means "Post No Bills," the phrase French cities stencil on walls to ban flyers and graffiti. The song turns that warning into its whole setting. Every evening, first at seven, then at eight, the light fades and the singer reads whatever has been scrawled near a meeting spot where the person she's waiting for keeps not showing up: an insult calling someone a coward, a slogan against the police, naive drawings, and, hidden inside a children's spelling game, a rude word slipped past the censors letter by letter.
Underneath all that graffiti is a much quieter story. The walls are covered in other people's noise while she waits alone, her heart getting sadder as the evening goes on. By the last verse she gives up and erases the one thing she had added to that wall: a heart she'd drawn for someone who never came.