The numbers indicate the betting levels of the game. 10/20 means all the bets and raises are in increments of $10 or $20. In 7-card stud, usually the first two betting rounds are the lower amount ($10 in this example) and the last 3 betting rounds are the higher amount ($20 in this example). In Texas hold 'em, the first two betting rounds are the lower amount, and the last 2 betting rounds are the higher amount.
When you see three numbers, such as 5/10/20, this means there are three different amounts for different betting rounds. This is uncommon, but not unheard of. Since these can vary from place to place, the easiest thing to do, prior to playing, is to ask a dealer or floorperson at the room how the betting works. These games tend to be wilder, because the entry bets are a small fraction of the betting limits at the last card. This often inspires people to play any hand, knowing they can get a big return on investment if they hit a good hand.
