Out of character (or OOC) activity by a player is any activity (e.g. talking) or knowledge that is not in character, meaning the activity is not considered to represent the activity of a character in the game world. Out-of-character talking can be about the game being played, even including interacting with the game's system, but can also include table chatter about other things.
Out-of-character knowledge generally refers to information that a player knows but the character does not. Making choices for a character based on out-of-character knowledge is the definition of metagaming.