According to the captain, he briefed the flight attendants prior the flight that it would be bumpy since they were flying around thunderstorms. While descending through 15,000 feet, the airplane encountered moderate turbulence for several seconds. According to a flight attendant, they were informing the passengers of their gates for the next connection in the destination terminal, when the airplane encountered the turbulence. One of the flight attendants sustained a serious injury, while the other was not injured. The airplane was not damaged and landed uneventfully at the destination airport.