Session zero refers to the pregame planning session that often occurs prior to starting a table top role playing game campaign. This serves two main functions. It allows the game master to explain what he or she is planning to run and it lets the players create their characters. Depending on the complexity of the system played, everybody's familiarity with it, the groups level of interpersonal familiarity, the level of specificity called for by the GM's explanation, and a score of other things too idiosyncratic to express this can take anywhere from ten minutes to several hours. The complexity of character creation varies wildly from system to system and the constraints on party composition are often decided by what the GM wants to run. In typical Dungeons and Dragons the fighter/rogue/wizard/cleric roles being filled will typically suffice for the purpose of most games. In contrast, a Vampire the Masquerade game has physical, mental, and social stats the importance of which are very contingent on the campaign planned. Players will typically try to create characters with non overlapping skill sets to maximize the groups versatility but can always try and create highly specialized and targeted parties.
Character creation aside, this is a time for new groups to get to know each other out of character and establish what they want from each other and the game. The game master can establish what is and isn't okay in the game from player versus player interactions to evil player characters. Players can establish what they want like the opportunity to build a base and buy real estate in the game world and things they don't want in the campaign like drug use or harm to children. Characters' back stories can be arranged to intersect so that there's more to the group's origin than "you all met in a tavern." More important than all of the details is everyone having the same basic idea of what they're doing. Oh, and scheduling. Got to know when the game is actually going to happen.
While session zeros aren't strictly necessary they have become fairly standard in the hobby. The difficulty of coordinating gamers makes it a good idea to block out a full game session just to get all of the rigamarole involved in starting a campaign out of the way rather than try and deal with it as it comes up. On the other hand session zeros can drag very badly where decision paralysis takes root. It's as important (if not more so) to maintain forward momentum here as in regular sessions lest the whole endeavor fail to begin. Game Masters who choose to have a session zero need to go in with a clear vision and communicate it effectively for this to have the desired effect.
IRON NODER XV: LAST SECOND BARE BONES IRON NODER FREAKOUT!