Ici-bas literally translates to "here below" or "down here". It's a slightly more poetic and evocative way to say "here on Earth" or "in this world", often carrying a connotation of earthly existence in contrast to an afterlife.
In the context of "Mon Ami", the word is used as the singer imagines seeing their departed friend "ici-bas" (here below), implying that even though the friend is gone, their presence is still felt in the living world. It adds a touch of melancholy and spiritual reflection to the lyrics.
"Mon Ami" is French synth-pop band La Femme's song for a friend who died young. Instead of dwelling on the loss, the narrator insists on his continuing presence: "Mais tu sais parfois je te vois ici-bas" ("But you know sometimes I see you down here"), and the recurring line "On te regarde depuis ta tombe qui nous sourit" ("We look at you from your grave which smiles at us") turns the grave into something gentle rather than final.
The second verse fills in who he was — barely twenty, his ideas still half-formed, gone "Bien trop longues pour ce monde" ("way too long for this world"). The song closes on three repeats of "Tu es là, encore" ("You are here, still"), holding him in the present tense rather than the past.