A slot machine reel means the spinning cylinder on which the symbols are displayed. Think of reels as the spinning squares you see when you hit the button. Traditionally, slot machines had physical reels that spun around when activated. These reels would come to rest on a series of symbols, and the combination of these symbols across the reels determined whether you won a prize.
In modern video and online slots, these reels are typically simulated on a digital screen, but they function in much the same way. The symbols on the reels are randomized for each spin, and the combination of symbols that appear when the reels stop spinning determines the payout, if any.
Classic slot machines usually have three reels, but modern and video slots often have five, and sometimes more. Each reel usually displays three or more symbols at a time when the spin ends, but this too can vary between games.