A royal flush is a straight flush composed of the highest cards, such as 10? J? Q? K? A?. But of course they don't all have to be in order. Q? 10? A? J? K? is still a royal flush. The Royal Flush is the jackpot in video poker, and comes around about once out of every 40,000 or so hands -- or a week and a half of full-time play. Hey, it could be worse: The jackpot on a typical slot machine only hits about one out of every 262,144 spins.
Despite the importance of finding the best machines, most players don't. That's why casinos can offer both decent and lousy machines in the same casino and be confident that gamers will still play the lousy ones. They have to keep some good machines, otherwise they'd lose all the players who know what they're doing. But most of the machines will be bad, and most gamers will play them anyway. Heck, in Vegas even casinos and supermarkets have video poker, with absolutely terrible paytables, but people will still play them rather than going across the street to a casino where they can get seven times better odds. Go figure.