Miss Bessie's "Bag"?

Hello,
In the book, there are references to Miss Bessie's "Bag"?

What is this bag exactly?
Hefre are the paragraphs that mention it:

"All right, Jesse. Get your lazy self off that bench. Miss Bessie's bag is probably dragging ground by now. And you still got beans to pick."

"It was nearly dark. Miss Bessie's bag was tight, and she was fidgeting with discomfort. She should have been milked a couple of hours ago. He eased himself onto the stool and began to tug; the warm milk pinged into the pail."

It seems to have something to do with milking.
Can anyone explain to me what this is?
Thanks.