Gambling is the practice of engaging in games of chance with money at stake. This can be as simple as betting on the outcome of a fighter's bout, or as complex as many medieval dicing games. Gambling was as popular in period as it is today, although the modern obsession with playing cards was replaced with dice as cards were not invented until the 14th Century.
Gambling in the SCA
Gambling for large amounts of money in the SCA is frowned upon. Some people are uncomfortable with gambling, and there are, after all, places where one can go to do that sort of thing without ruining the game for everyone else. If you want to gamble in period fashion at an SCA event, it is probably best to keep to penny-stakes or SCA coinage.
Note: Gambling addiction is a serious problem for some people. If you know someone with a gambling problem, try to get them some help. The Ministry of Health (or local equivalent) can provide assistance to those struggling with addictions.