Mooncakes are a staple. It's a round pastry and there are many different fillings.
月饼是必须的。它是种圆圆的点心，有很多不同的馅料。

Staple
基本，必须

Turkey is a staple at most traditional Thanksgiving dinner.
大多数传统感恩节晚餐里火鸡都是必须的。

Music is a staple at parties.
聚会中音乐是必须的。

Mooncakes are a staple.
这里Mooncakes表示一类，这一类东西都必不可缺。

Fruits are an important part of a healthy diet.
水果是健康饮食中不可或缺的一部分。

Vitamins are a waste of money.
维他命就是在浪费钱。

Staple
订书针

Stapler
订书机

It's a...and there are...
如何描述一种食物

a. It's a round pastry and there are many different fillings.

round pastry
圆圆的点心

filling
馅料

Dim sum
点心/早茶

b. Dim sum is a Cantonese style cuisine and there are many steamed dishes like barbecue pork buns and shrimp dumplings.
点心是一种广式菜，里面包括很多小吃比如叉烧包和虾饺。

c. Hotpot is a pot of soup and you cook meats and veggies in it.
火锅就是一锅汤你可以在里面涮肉和菜。