Trouillards is a colloquial and somewhat strong term for "cowards." It comes from the word "trouille," which is slang for fear or fright.
In the song, France Gall uses this word to contrast the free-spirited piano player with those who are afraid to be themselves: "Quand les trouillards sont à genoux" (When the cowards are on their knees). It's an engaging word that highlights the song's theme of courage and individuality.
France Gall's "Il Jouait Du Piano Debout" is a celebration of individuality and the courage to be different. She sings about a boy who "Ne vivait pas comme les autres, c'est tout" [didn't live like the others, that's all], and questions why "Les gens qui n'sont pas comme nous / Ça nous dérange" [people who aren't like us / bother us]. It's a powerful message about accepting those who choose their own path.
The heart of the song is in the repeated line, "Il jouait du piano debout" [He played the piano standing up]. For France Gall, this isn't just a small detail; it means "beaucoup" [a lot]. It means "qu'il était libre / Heureux d'être là, malgré tout" [that he was free / Happy to be there, despite everything]. She sees him as a symbol of freedom, especially when "les trouillards sont à genoux / Et les soldats au garde à vous" [cowards are on their knees / And soldiers at attention]. He simply wanted "être lui, vous comprenez" [to be himself, you understand]. It's a beautiful song about staying true to yourself, even when it makes others uncomfortable.