"Op" stands for operator and means full administrative permissions for everything on your server.
To become op, you must use the Multicraft console. Login to Multicraft and then click on the Console link.
PLEASE NOTE: Multicraft console commands do not require (or work) with the leading /
Once you're on the console, just type op YOURMCUSERNAME to become operator. Then you can use all commands in-game while playing on your server.
To grant another player OP type: /op MINECRAFTUSERNAME
Example: If the player's name is VG4life, you would type: /op VG4life while in game or just op VG4life in the Multicraft console.
To DEOP a player type: /deop MINECRAFTUSERNAME
Example: If the player's name is VG4life, you would type: /deop VG4life while in game or just deop VG4life in the Multicraft console.
Giving other players op privileges is dangerous and not recommended unless you trust them implicitly. You have recent backups, right?