Seille-o is a modern, informal slang term for "money". It's a fantastic example of verlan, a type of French backslang where a word's syllables are inverted to create a new one.
This word is the verlan of oseille, which is also a popular slang term for money (its literal meaning is "sorrel", a type of herb). In the song, the lyric "Elle a vu toute ton seille-o" means "She saw all your cash," suggesting someone is only interested in the main character for his wealth. Learning words like this is key to understanding modern, spoken French!
Marwa Loud's "Bad Boy" is a sharp look at a guy who's trying way too hard to live a certain lifestyle. She calls him out right from the start: "T'as que des bad girls ouais t'es qu'un bad boy" (You only have bad girls, yeah you're just a bad boy). She sees through his act, pointing out that he's all talk. She tells him, "T'as pas un euro, fais pas de deal / Et t'as pas de kilo fais pas le dealer" (You don't have a euro, don't make a deal / And you don't have a kilo, don't act like a dealer). It's a blunt and funny takedown of someone pretending to be a big shot.
While she's observing this scene of fake friends and girls who are only interested in money, the singer also reveals her own feelings. She's tired of it all and wants to get away, singing "Moi j'voulais m'en aller / Et ce depuis ouais tant d'années" (I wanted to leave / And have for so many years). She feels stuck in a world where everyone seems unlucky, repeating the phrase "On n'a pas, pas d'bol" (We have no, no luck). The song describes a cycle of trying to impress people, spending money you don't have on girls who will leave you with the bill, and feeling trapped in a life you can't escape.