Updated since 1.0.7
Prelobby (waiting room or main world) is a place where all players appear on join if game is not running. Here you can right click any block to open /options menu. Here you can select team, kit, set your hotbar in hotbar editor and vote for maps without using /v #.
Team selection is simple - click on your desired team. By default there is Default team (or Random if there are more teams than 1) - select it to play, and spectator team - select it if you don't to play next match. You have to reselect team after each match.
Kit selection is same, except specific games can save your latest kit (using MySQL database, devs note: use player_set_latestKit(player, kit)) or disallow players to use some kits (devs note: use player_hasKit(player, kit)).
Hotbar Editor (HotbarEdit, HBE) is a way to set items to your liking in hotbar (it resets after match by default, devs are recommended to replace this with MySQL implementation in the game code or OMGPI fork). To swap slots, click one slot and item will disappear, click another and they will swap. Items shown in HBE are from .kit file, find out about Kits in another docs section.
If there is enough players (more than gamefig value minPlayers), countdown starts. On 10 seconds left, it loads a map based on votes. After countdown everyone teleported to gameworld. When game match is finished (someone won), OMGPI systems unregister and loading cycle repeats.