Sorry babe, I have no clue what a 'peplum' is

I don't get how a "medium purple dress with a cute light purple belt" gets turned into "rainbow fucking circus dress"

it's when there's a stiff waist that sticks out

The dress on the left is clearly super boring and useless. The dress on the right is glorious.

What the heck is a peplum? PS, I'm a girl.