IC means, simply, in-character.
The IC grid, for example, is the area on the game where Manhattan and surrounding areas have been built. When using @emit and say in this area, whatever you pose or say is considered to be in-character. If you wish to talk to someone in these areas in an OOC manner, you must use the 'ooc' command or private pages to indicate that it is the player and not the character talking.
Anything your character does or says while roleplaying is considered IC. Any plots run by staff or players is considered IC. Ultimately, anything that effects characters rather than players is considered IC.
See also OOC.