Hello! False protagonist is basically a kind of plot twist--
It's a character who we think is the protagonist due to the story mainly focusing on them, but as the story progresses, it's revealed they're not the protagonist. There's multiple ways authors can reveal this (Killing them off, making them the antagonist, making them less important plot-wise etc.). After the reveal, they get replaced by the ACTUAL protagonist and the story starts to focus on that person from there on.
It's fun! One of my favorite type of plot twists ever~