By default, if your bot doesn't understand a message, it won't reply to it.
If you wish to add a custom reply for those messages, go to the Smart Responses>Trained responses section, click on "Any Unrecognized Message"
Enable autoresponse here:
Simply add your response or a few responses the bot will choose between. These will be sent to your followers when the bot doesn't understand a message.
*If you just enable this option and don't add any custom responses your bot will display this ❤️ emoji for all messages it doesn't understand.*
In these response variations, you can also add images, audio, video, carousel, an external link or link to your features.