Treillis refers to military-style camouflage clothing, also known as fatigues. It's a specific and evocative word that you likely wouldn't learn in a typical French class.
In the song, Orelsan uses this word to create a powerful contrast. He describes men in treillis (soldiers) holding hands and playing in a circle, a childish and innocent image that clashes with the harsh reality of war the word implies. This clever juxtaposition is central to the song's theme of masking difficult truths.
On the surface, Orelsan's "Tout Va Bien" sounds like a sweet song to reassure a child. He keeps repeating the title phrase, which means "Everything's fine." But listen closely to what he's explaining. The song is a masterclass in dark irony, where an adult tries to explain the world's horrors to a kid in the most innocent way possible.
For example, if a man is sleeping outside, it's because "c'est qu'il aime le bruit des voitures" (it's because he loves the sound of cars). If a woman has bruises, it's because "c'est qu'elle a joué dans la peinture" (it's because she played with paint). And if buildings are exploding? That's just to "fabriquer des étoiles" (make stars). The song is all about the gentle lies we tell children to protect them from a world that isn't always fine at all.