Hirondelle translates to "swallow," referring to the bird. It's a beautiful and somewhat less common word in everyday French, making it stand out in the lyrics.
In the context of the song, Brel uses "le vol d'une hirondelle" (the flight of a swallow) as an example of the simple, beautiful things we should remember to see amidst life's difficulties. It evokes a sense of freedom, grace, and hope, contrasting with the harsh realities described elsewhere in the song.
Jacques Brel's "Il Nous Faut Regarder" is a powerful reminder to look beyond the obvious difficulties of life and find beauty and hope. He starts by describing the ugliness we see around us: "Derrière la saleté s'étalant devant nous" [Behind the filth spreading before us], "les yeux plissés et les visages mous" [creased eyes and soft faces], and even "les frontières qui sont de barbelés" [borders that are barbed wire]. But then he urges us to look further, to "regarder ce qu'il y a de beau" [look at what is beautiful].
What does he suggest we look for? Simple, lovely things like "Le ciel gris ou bleuté, les filles au bord de l'eau" [The grey or bluish sky, the girls by the water], and the comforting thought of "L'ami qu'on sait fidèle, le soleil de demain" [The friend we know is faithful, tomorrow's sun]. He also tells us to listen, to ignore the "concert des sanglots et des pleurs" [concert of sobs and tears] and instead hear "l'oiseau au fond des bois" [the bird deep in the woods] and "Les berceuses des mères, les prières des enfants" [Mothers' lullabies, children's prayers]. It's a beautiful call to appreciate the quiet joys and enduring hope in the world, even when surrounded by hardship.