I think it's related to the term 'period' as the end of a sentence to make it more emphatic, like a 'full stop'. It's becoming more common to see in in Ebonics or African American vernacular except it's usually spelt incorrectly as 'periodt' to mimic a stereotypical black woman's accent. (E.g.: Das right gurl, I said what I said, PERIODT!) In this case, the meme or the challenge is asking women to accept and love their bodies by matching their body shapes to letters of the alphabet in that word.