To multiclient is to, through one of a few means, use multiple Tibia accounts at the same time. This used to be prohibited but is no longer against the Tibia Rules. Note that the use of modified clients is still a rule violation.
It is also known as MC. It is done by either using a program that lets you have more than one Tibia client open at the same time, using a modified Tibia client that allows for multiple clients to be open, using multiple computers, or multiple browser instances running the Tibia Flash Client.
These programs and modified clients are usually made by people who intend to steal account information. There are many incidents of mass account hacks after an MC creator, who has got enough people to use his program, can use a back-door trap and take the cheaters' account information. The benefits do not outweigh the risks; besides taking away the fun from the game. MC creators usually work via a special program, or with a simple hex editor where a few lines of code are switched to allow such behavior.