Aérosol translates directly to "aerosol" in English. It's an interesting word because it's a direct cognate, making it easy for English speakers to recognize, yet it's used in a surprisingly metaphorical way in the song.
The lyrics state, "C'est comme de la confiance en aérosol" (It's like confidence in aerosol). This creates a vivid and somewhat quirky image of confidence being something that can be sprayed on, highlighting the speaker's unique perspective on how closing his eyes helps him gain self-assurance.
Have you ever closed your eyes while singing to look cool, only to forget the world around you? In "Ouvre Tes Yeux Simon !", Quebec pop-rock band Les Trois Accords turns this silly habit into a hilarious debate. Everyone is shouting at Simon to look up with "Ouvre tes yeux quand tu chantes" ("Open your eyes when you sing"). Simon defends himself with funny, specific excuses: shutting his eyelids gives him "confiance en aérosol" ("spray-can confidence"), lets him dream up wild characters, and above all, helps him remember his lines ("me souvenir de mes paroles").
Eventually, Simon realizes that keeping his eyelids shut means missing out on the real show. He admits that great stories happen right in front of him, not just in imaginary worlds. When he finally looks out, he sees friendly faces, catches eye contact, and spots old classmates in the crowd ("des garçons que j'ai connus à l'école"). By the end, the connection with the audience feels so good that he proudly belts out "Je chante avec les yeux ouverts" ("I sing with my eyes open"), no longer caring if he forgets a word or two.