The image is of a meme that features a package of chicken drumsticks placed on a store's meat display tray. At the top of the image, there's a text overlay in bold white letters that reads, "I had a blast at the store today. I asked the young checkout girl if these were front or back legs."
The humor in this meme emerges from the absurdity of the question posed by the customer. Chicken drumsticks, as commonly known, come from the lower leg of a chicken, which is, in essence, the equivalent of the back legs. Chickens are bipeds, meaning they walk on two legs, so they do not have front or back legs in the way quadruped animals do.
The second part of the text cements the joke: "She didn't know and said I'll go ask. When she got back she said, 'Not funny.'" This further builds on the humor by suggesting the checkout person took the question seriously enough to ask someone else for clarification, indicating they might be unaware of the chicken's basic anatomy.
People might find this meme funny due to the play on expectations and a sort of innocent misunderstanding. The idea that a grocery store employee could be uncertain about something so universally known as the fact that chickens only have two legs tickles the funny bone through situational irony.
Another layer of humor could be derived from the deadpan response the checkout girl reportedly gave upon her return. Her "Not funny" remark is humorous because it suggests I HAD A BLAST AT THE STORE TODAY I ASKED THE YOUNG CHECKOUT GIRL IF THESE WERE FRONT OR BACK LEGS SHE DIDN T KNOW AND SAID I LL GO ASK WHEN SHE GOT BACK SHE SAID NOT FUNNY