Fus is a form of the verb être (to be) in a rare literary tense called the passé simple. You'll almost never hear this tense in modern conversation; it's reserved for storytelling, novels, and songs, giving the lyrics a poetic, narrative quality.
When Stéphane sings, "Joueur je fus" (A player I was), he's not just saying what he used to be. He's using this special tense to frame his past as a finished chapter in a story, adding a layer of dramatic finality to his reflections.
This song is a classic story of someone who doesn't realize what they have until it's gone. Stéphane starts by being almost comically detached when his partner leaves him. They say, "C'était ta dernière chance" [It was your last chance], and his first thought is about the practical stuff, like their cancelled "vacances à Dubrovnik" [vacation in Dubrovnik]. He's in total denial, thinking, "On peut pas me quitter" [You can't leave me]. It's all part of his self-image as a "Joueur" [Player], someone who runs from real feelings: "Joueur je suis, douleur je fuis" [I am a player, I flee pain].
But of course, the pain catches up. Suddenly the empty spaces in his apartment feel huge, and he's escaping into "le petit écran" [the little screen] to forget. The big twist comes in the chorus, where he admits, "C'est qu'après la fin / Que j'ai ouvert ma garde" [It's only after the end / That I opened my guard]. He only let himself be vulnerable after it was too late. The song ends on a bittersweet note. He's changed and grown from the experience, but he knows his ex won't get to see that new person. It's a sharp, honest look at regret and the hard lessons we learn from heartbreak.