IC stands for "in character", and OOC stands for "out of character". The differences between the two have a fine line, and they often come up within a role-playing game.
When you do things "ICly", it means your playing from your character's point of view, and determining their actions or reactions from their shoes, instead of your own. It is frowned upon to play characters based on your own feelings, such as acting them out in ways they normally wouldn't just to gain something you yourself would like.
It's when people play this way that it ruins the fun of the game for everyone else around them. If people simply do things out of character to gain things all the time, then the game becomes obsolete. It becomes all about hoarding items and resources, instead of creating a community, and playing a role as someone different than yourself. If you're just playing yourself, or playing yourself at times it becomes convenient and profitable, you're just making those around you frustrated.
It creates tension within a game where the very idea of playing a role should be fun.
For instance, if a character is normally shy, with-held, sensitive, and feeble, you wouldn't expect that character to jump forward at a moment's notice try to play leader because the player knows OOCly they can take their community to better resources elsewhere.
With that said, it's important to stick to your character's perspective. They may have some of the same values as yourself, and they may even be just like you because that's how they developed; but there is a difference between that, and using out of character information (such as the forums, or guide) inside the game when it benefits your character.