"单子"是一张纸，上面写着东西，比如你要买的东西，或者你要付的钱。

"单子" is a piece of paper with things written on it, like what you want to buy or the money you need to pay.

在饭店吃完饭，服务员会给你一张单子。

After eating at a restaurant, the waiter will give you a 单子.