Piges is a French slang term for "years." It's a casual and somewhat informal way to refer to age, similar to saying "I'm 21 years old" in English.
In the song, Zélie uses it when she sings, "J'ai 21 piges" (I'm 21 years old) and later, "J'ai 26 piges" (I'm 26 years old). This word is interesting because it's not the standard word for "years" (ans), offering a glimpse into everyday spoken French and adding a touch of authenticity and relatability to the lyrics.
Zélie's "Faut Que Je M'en Souvienne" (I Have To Remember) featuring Mauvaise Bouche is a really honest look at growing up and dealing with life's challenges. She starts by looking back at being 21, with "15 potes chanteurs, ma mère au bout du fil" (15 singer friends, my mom on the phone). It's a time of turning the page, even if there are "lourds instants" (heavy moments) where joy is hard to find. She writes about it, and that's a start!
As the song progresses, she fast-forwards to 26, and things feel heavier. The world is a tougher place, and she's writing songs while "y'en a qui s'prennent des balles" (some are getting shot). She's losing sleep and feeling guilty, realizing that "plus j'suis lucide, moins c'est facile" (the more lucid I am, the less easy it is). Even knowing how lucky she is isn't enough to bring peace. She uses songwriting to try and deal with her pain, but she knows "elle partira jamais" (it will never leave). It's a powerful and relatable journey of trying to hold onto who you are while the world keeps changing around you.