Output nodes are flow nodes that specify how the bot should respond. Most importantly, they contain the answer texts, but they also let you specify various other properties.
The most common thing to add to an output node is an answer text: a response from your bot in natural language. Answer texts are specified in the edit view in the output. You may add several answer texts that are phrased slightly differently. Having multiple variations of an answer text is mostly relevant when you expect that the flow containing the answer might be triggered several times in the same conversation. That way your bot won't sound repetitive if a user runs into the same output node more than once.
If you define more than one answer text, the final text will be used as a fallback, meaning it will be repeated after all the other answer texts in the output have been given once.