Bre-som is a verlan term, which is a type of French argot where syllables of a word are inverted. It comes from the word "sombre," meaning "dark" or "gloomy."
In the context of the song, when Landy says "Paris le soir c'est magique, Et Denis Saint c'est bre-som," he's contrasting the magical atmosphere of Paris at night with the "dark" or "shady" reputation of Saint-Denis, a suburb often associated with street life and illicit activities. It's an interesting word that reveals a common slang usage in French rap.
"Vie D'avant" ("Life Before") is Landy's reflection on how much has changed since coming up in the streets. The verses lay out the hustle in blunt terms: dealing, watching for the police, and building wealth fast enough to compare himself to a Rothschild. But every time the beat drops back into the hook, he keeps circling to the same thought - he can't stop thinking about the life he used to live, and he knows he could still throw the gloves back on if he had to.
The second half widens the picture: late nights, showing off jewelry, smoking to numb dark thoughts, and cruising through Paris at night while still watching his back in the 9-3 (Seine-Saint-Denis, a department north of Paris associated with tougher neighborhoods, not America's easy fantasy of the high life). The repeated warning to avoid "les glissades" (slipping up) in the ghetto is the song's real center: success hasn't erased where he's from, and one wrong move could send him right back to his "vie d'avant."