Among the many different kinds of online games, the most successful are the MMORPGs. The term stands for Massive Multiplayer Online Role Playing Games - games where a great multitude of players control virtual characters - known as Chars or Avatars for short - through adventures in fantasy worlds.
To those versed in the lingo, "4/6 lfm tank and dd" is a more scientific notation than lazy shorthand. It represents a want ad with very specific information: The first part of this seeming jumble of letters and numbers indicates that four characters have already come together for what is to be a six member group (4/6). They are looking for two more members to fill out the group (lfm - looking for more). Specifically, they are looking for either tank or dd (damage dealer) characters.