"Expanding reels" are a reference to a feature in online slots in which certain symbols on one or more reels can expand across the entire reel to conclude a winning combination. Such expanding reels can lead to bigger payouts and increase your chance of winning. In some cases, the expanding symbol can even end up filling up the entire reel, allowing for even bigger wins. The expanding reels serve as a popular bonus feature in many modern online slots and can spice up your gaming experience significantly.