That's the seal of Muhammad. I've been into flags for a long time so basically the moment Daesh started showing up in the news when I was maybe 13 or so I was already hearing about the symbolism of its flag, including that the little circular symbol was the prophet's seal, which I believe reads "Allah's prophet Muhammad" -- as opposed to the Shahada which says "Muhammad is the prophet of Allah". The reason for the flipped word order compared to the Shahada is so that Allah's name is literally above Muhammad's, which I think is pretty cool.

All in all the seal of Muhammad is a pretty striking, interesting, historic symbol, but now most non-Muslims only really associate it with Daesh or similar groups. I was at the library recently and saw a book called To søstre whose cover was literally just the seal of Muhammad in order to convey that it was a book about Daesh (specifically about two teen girls who were radicalized into joining). I thought that was a poor choice of cover, that played into basically what ignorant non-Muslims associate with the symbol rather than what the symbol actually means.

Edit: OK so the word order isn't actually reversed from that on the Shahada per se, it's just that you read the words in reverse order.