A GM or Game Master is a player that has special abilitys in game that other players do not. Their abilitys are chosen by the server owner/admin with letters. Each letter has thier own set of commands that the GM will be able to use. GM's are mean to help others in-game and to host events and such.
Most private servers wont allow just anyone to be a GM because they want and experienced GM for better help.
Here Are some of the commands a GM could use:
.Revive - Revives selected player or yourself.
.summon <player name> - Summons anyone online to you position.
.modify level <#> - Changes the level of any player to any level.
.modify speed <#> - Changes the speed of any player up to 255 and thats pretty much light speed.
.cheat fly - Allows the selected person to FLY.
Hope this explaned what you needed.:smile: