In it’s simplest form, metagaming (also known as meta) is using informations outside of your characters life to use them as guideance, and it’s highly punishable, as you can get multiple weeks of punishments and even a permanent ban if it continues. Most usually, metagaming is used for getting help if someone attacks you or if your town is getting raided. The most usual forms of metagaming can be found on Skype, Teamspeak or forums. It can also happen with the servers Out-Of-Character messaging plugin, where people simply message other people out of character using the /msg command.
There are many more aspects of metagaming that are done in the game. Mostly it concists of the character knowing something that he would not know. Knowing someone's name without being told, hearing people talk even though they are underground or rooms away, knowing that a character is a Frost Witch, Homunculus, Wraith, or any other creature just because you read their card, remembering the events that lead to the character's death (characters can be revived, but they can't remember how they died), having information that is hidden in game (such as members of a dark guild), seeing mc name tags through walls and heading to the player (if the player doesn't want being randomly found, if he is just waiting for something to happen, or someone to roleplay with, it's alright), reading an emote of an action that your character wouldn't see and reacting to it, and many more.
Be aware of it, and don’t join the people participating in metagaming, as it’s much more pleasurable to simply enjoy roleplaying without the effects of Out-Of-Character (OOC) information! This of course does not mean that you should stay away from the forums, as the forums can provide tons of both useful info and additional roleplay to be had. But just restrain yourself from using OOC info as your help when your character is making a decision.