주세요 after a noun means please give me: 이거 주세요. Added to a verb stem as -아/어 주세요 it means please do this for me: 보여 주세요. And -(으)세요 on its own is a polite request or instruction: 여기 앉으세요.
Examples
- 더 큰 사이즈로 주세요.
- Please give me a bigger size.
- 이 옷을 입어 봐도 돼요?
- May I try this on?
- 저기에서 기다리세요.
- Please wait over there.
The mistake to avoid
주세요 alone asks for a thing; -아/어 주세요 asks for an action.
Where this fits in
쇼핑과 비교, Korean A2: Shop, ask about prices and sizes, pay, and compare two things.