A Short Code is a 5 or 6-digit number (e.g., 12345) used to send to and receive SMS messages from mobile phones. Short Codes are allowed for application to person (A2P) communication, which is why businesses who need to communicate to customers on a large scale can only use Short Codes (i.e., not Long Codes). Plivo offers Dedicated Short Codes.
See What is the difference between a Short Code vs a Long Code? for more details.