The response system has been on bots for ages and has been some of the most simple features of a bot. You input a trigger word/response pair and whenever somebody says the trigger word in usually any room, the bot outputs the response in the same room. However, the user who says the trigger doesn't always know that the bot is going to respond.
Because of some responses to be triggered by just ordinary emoticons like :O, :D and :), the response feature can often be seen as annoying if you don't use it "correctly".
Here's an example of a response with the trigger 'hey' and the response 'Why hello there!':
<user>hey <bot>Why hello there!