Expression/ idiom meaning: "Hold your shoulders back!"

I was reading the poem Father Forgets (pdf) by W. Livingston Larned.

Part of the line said, "Goodbye, Daddy!"

I frowned, and said in reply, "Hold your shoulders back!"

I couldn't find the exact meaning of this phrase.

Here's the part of the poem:

At breakfast I found fault, too. You spilled things. You gulped down your food. You put your elbows on the table. You spread butter too thick on your bread. As you started off to play and I made for my train, you turned and waved a hand and called, "Goodbye, Daddy!" I frowned, and said in reply, "Hold your shoulders back!"

Solution 1:

The meaning is literal. It seems the father has been repeatedly finding fault with his child, and instead of returning the friendly "Goodbye" he criticises the child's posture. You pull your shoulders back when you are standing up straight. I was often admonished to do so when I was young.