L'abri means 'shelter' or 'refuge'. It's often used in phrases like "à l'abri de" (sheltered from).
In the song, it appears in the opening line, "À l'abri du soleil dans les rues de leur ville" (Sheltered from the sun in the streets of their city). This word sets a tone of protection and perhaps a hidden existence, hinting at the characters' impending departure and the secrets they hold.
ZAZ's "Avec Son Frère" tells a really touching story about two brothers who are about to leave their home behind. The lyrics paint a picture of them trying to hold onto every last memory of their city, from the smells and colors to the feeling of the sun. It's clear they're making a huge sacrifice, as the song says, "l'avenir a un prix avant que d'être un choix" [the future has a price before it's a choice].
There's a lot of quiet emotion in their goodbyes. They have a secret from their father and a promise to their sister to protect each other. The most heartbreaking part might be the letter for their mother, simply saying "Pardon, merci" [Sorry, thank you]. It really makes you feel the weight of their departure and the love they're leaving behind for a "pays moins chaud" [less warm country].