If you get something for free that would normally have to be bought, especially in a bar or restaurant, it is called “on the house”. 在酒吧或者在餐厅里有时候会听到店员说“It’s on the house”，事实上他们要表达的是不需要付钱，这个由店家请客。
 
Examples:
This hotel serves an afternoon tea. And that’s on the house. 这家旅馆免费提供下午茶。
I was the ten thousandth customer, so my dinner was on the house. 我是第1万名顾客，所以我的晚餐免单。
We had to waiter for a table so they gave us the lemonade on the house. 我们要等桌，所以店家免费提供了柠檬水。