What does the `里` mean in `这些狗里有一只穿红色的毛衣`? In the following sentence:

这些狗里有一只穿红色的毛衣


What does the 里 mean? I think the sentence means something like The dogs wear red sweaters. But why does it require 里 there and what meaning does it add?

I'm learning about simplified mandarin language, but would like to know if it differs from the traditional in how it is used.
It means one of these dogs wears a red sweater. ...里 literally means in/among ..., and it is followed by 有一只, which means there is one. Hence the whole sentence translates literally as Among these dogs there is one wearing a red sweater.