To kidnap someone is to abduct them, that is, remove them from a place against their will by force or threat of force. In a best-case scenario, the kidnapper then ransoms the victim, who is returned safely.
Kidnappings were common in period, especially when the victim had influential friends or family. Other victims of kidnapping were enslaved. Women were particularly vulnerable to kidnapping. Sometimes, the victim would be raped and made pregnant in order to force marriage. In other cases, as happened to Eleanor of Aquitane, a priest could be induced to wed the kidnapper and his victim against the victim's will.
There is some belief that the practice of having bridesmaids at a wedding stems from an attempt to deceive any would-be kidnappers from snatching the bride on her way to the altar, but there is little evidence to support this.
"Kidnapping" in the SCA
It is important to note, however, that kidnapping is a major felony in all Western countries, and comes with serious consequences. Should you "kidnap" someone in fun for ransom at an SCA event, it must be made absolutely clear to everyone, especially the "victim", that this a part of the game. No one should be "kidnapped" without their consent.