SMS shortcodes are reserved 5-6 digit numerical codes that can be used with SMS services like Twilio (which powers the GiveWP Text-to-Give add-on).
Currently GiveWP does not support shortcodes. This feature request is here so that folks interested in that can make their votes and comments heard.
Note that to own a shortcode through Twilio, there is currently an application process and it costs around $1000 USD per month to own that shortcode, not counting the initial fees to set it up. That's part of the reason why GiveWP doesn't currently support them, but we're open to feedback in the form of votes and comments here.