In computing, a boolean (bool) is a data type that has exactly two possible states, usually referred to as True or False. A boolean is sometimes referred to as a flag. See the wikipedia article for more general information.
In the GECK there is no specific data type for booleans, but in a boolean context, such as an if statement, the value 0 (zero/an unassigned ref variable) is considered 'False', whilst everything else is considered (reduced to) 'True'.
If a function description states that it returns a boolean (bool), it will return the number 1 for true, or 0 for false. This could be stored in an int variable if desired.