This powerful phrase literally means "the piece that was missing." It's often used metaphorically, like saying someone is the "missing piece of the puzzle."
After describing his difficult past, the artist declares "ahora en este juego, soy la ficha que faltaba" (now in this game, I'm the missing piece). He uses this metaphor to show he's gone from being overlooked to being the essential, game-changing player. It's a memorable expression of self-worth and success.
6ix9ine opens this track by saying he wants to confess his sins and tell a story no one has heard before. He looks back at a tough childhood, where his parents separated when he was seven and there were far more bad times than good. He describes a life of poverty with powerful images, saying he never had toys and Santa never brought him gifts. Instead, he had "una guitarra sin cuerdas / Y un micrófono de palo" (a guitar without strings / And a stick for a microphone).
He contrasts that past with his present life of fame and money. People who used to ignore him now watch his every move. He says he used to be the one who always lost, but now he's "la ficha que faltaba" (the missing piece) in the game. Despite the success, he feels like "el mismo bobo con dinero y fama" (the same fool with money and fame) and asks his late father, who he believes is watching from above, only for health and blessings for his family.