You might want to check the comref.
The summary is:
kbAddTopic adds a topic (duh!) to a person with the .bikb (list of sentences), .fsm (conversation flow script for the AI) and/or .sqf (conversation flow script for a player) files as its parameters
kbTell makes the person actually say a specific sentence (defined in the .bikb file)
Conversation flow scripts are executed every time a sentence was said to the person with passed parameters being:
_sentenceId: the sentence as defined in the .bikb file
_topic: topic name registered via kbAddTopic
_this: the person that was talked to (receiver)
_from: the person that talked to the receiver and triggered the script