You use that expression especially when you do someone a favor.

” 。 尤其是当你帮某人一个忙时， 你会使用这种表达方式。

And " to do someone a favor" means that you are helping them.

而“to do someone a favor”意味着你在帮助他们。

To do someone a favor means to do something for that person, something nice for that person usually.

do someone a hav意味着为那个人做某事，通常是为那个人做一些好事。