If a user starts a new conversation with your bot they will by default see your page's description before pressing Get Started. You can replace it and have your page display a custom greeting, instead. This is a great tool for communicating your bot's functionality.
To set the Messenger Greeting you need to head to your Facebook Page → Settings → Messaging and scroll to the very bottom where it says Show a Messenger Greeting. There you can then toggle the button to Yes and change the text to your liking. You can also use personalisation like e.g. showing the first name of the user.
If you are interested in more information and examples on welcome and greeting messages you can find them here.