Immensité translates to "immensity" or "vastness". It's a beautiful and evocative word that paints a picture of something boundless and overwhelming.
In the context of the song, the narrator, grieving the loss of a friend, wonders if he will ever see his friend's kind smile "parmi l'immensité" (amongst the immensity). This use of the word suggests the vast, perhaps lonely, expanse of the world or even the afterlife, making the friend's absence feel even more profound and the hope of reunion more poignant.