It isn't exactly known what the exact symbolism is for her mentioning of teeth in songs, one theory is that it could be a reference to Zadie Smith’s novel called White Teeth, which details class divides and materialism, things which Lorde discusses often. Though, when a fan asked "My roommate and I have noticed that you talk about teeth a lot on your album. Why is that?", Lorde responded "just a weird obsession that has been with me always. ironically, i hate dentists."
"Royals" - Teeth are mentioned a number of times in this song, in the lyrics "I cut my teeth on wedding rings in the movies", and "But every song's like gold teeth, Grey Goose, trippin' in the bathroom".
"Team" - Lorde mentions teeth when she sings "A hundred jewels on throats, a hundred jewels between teeth" this can refer to braces which have joints that could be mistaken for jewels.
"Green Light" - Lorde sings "Those great whites, they have big teeth, hope they bite you" and "All those rumors, they have big teeth, hope they bite you" in the refrains. And after a fan commented to her on Twitter saying "wait there are so many teeth references in ph bc dreams where ur teeth fall out mean u have a fear of aging and that's basically ph" Lorde said "teeth are a big aging motif generally our lives are marked by the moments we gain and lose our teeth " which contradicts her earlier comment saying that it was "just a weird obsession".