Usta technically equals "mouth" (oral cavity = jama ustna), that's why you can say "She has her fingers in her mouth" - "Ona ma palce w ustach".
But it's very often used to refer to lips, for example you rather won't talk about applying lipstick on 'wargi' (although that's actually the word for lips: sg warga, pl wargi), but on 'usta'. And here it's a situation like that. It's used for lips a lot more often than the word 'wargi' itself.
Oh, also "usta" in any meaning is always plural, check here for clarity.