Fensterglas literally translates to "window pane" or "window glass". It's a compound noun that paints a specific, tangible image.
In the song, it sets a melancholic scene: "Kennst du das? Fensterglas / Nicht einschlafen können in 'ner fremden Stadt" (Do you know that? Window pane / Not being able to fall asleep in a strange city). This word evokes a sense of quiet observation and introspection, making it a unique and evocative choice for a song about urban solitude and longing.
MAJAN's "Taxi" is about the moment right before you actually leave someone, when the ride is already there and you still can't make yourself get in. The taxi pulls up outside, but she doesn't move (Das Taxi kommt, doch du steigst nicht ein — "The taxi comes, but you don't get in"). Instead she calls him from some strange city, too wound up to sleep (Nicht einschlafen können in 'ner fremden Stadt — "Can't fall asleep in a strange city"), and admits the pattern out loud: she keeps coming back, and always too late (Und ich komm' wieder / Wieder mal zu spät — "And I'll come back" / "Too late again").
By the second half the tone turns colder and more numb — her face has gone the color of ice (Nicht klar im Kopf, Gesichtsfarbe Eis — "Not thinking straight, face pale as ice") — and she genuinely can't tell whether the danger just passed or ended a long time ago (Is' das gerade noch, oder schon lang vorbei — "Is that just now, or already long over"). It's a song about circling back to someone you can't quite leave and can't quite stay with either.