Macaques refers to a type of monkey. It's an unusual and striking word to find in a song, especially in this context.
Gainsbourg uses it to describe a shocking and somewhat surreal scene: "Elle était entre deux macaques" (She was between two macaques). This unexpected imagery immediately grabs attention and hints at a wild, perhaps debauched, situation, making it very memorable and enticing for a learner.
"Flash Forward" plays out like a half-remembered dream Gainsbourg can't shake. A stranger knocks on a door at night with no warning, "Je frappe à ma porte, toc toc" (I knock on my door, knock knock), and pushes past a hammock creaking in the dark toward a scene that gets stranger with every line, a woman "entre deux macaques" (between two macaques) at something like a Woodstock crowd, a camera catching it all on film.
The title is the joke: what feels like it's rushing forward is really a flashback he can't escape. He ends up questioning his own grip on reality, "Qui moi paranoïaque?" (Who, me, paranoid?), asking the hotel's night porter to vouch for him. Whatever happened that night is burned into memory "à jamais sur le bloc notes" (forever on the notepad) of his mind, and he tells us it will keep resurfacing "en flash back, bordel" (in flashback, damn it) until the day he dies.