A short code is a 5 or 6-digit number that can send and receive SMS (and MMS) to and from mobile phones. Short codes are recommended for high volume applications.
Twilio Short Code Offerings
Twilio currently offers short codes leases in 3-month or 1-year increments in a variety of countries. Short codes may be a random number (in the US), or a vanity number that you pick; A vanity short code is a specific number that you choose, while a random short code is assigned to you and can’t be reviewed in advance. We also support programmable messaging services on customer leased short codes.
Short Code Messaging
Twilio short codes can send 100 SMS messages per second by default, but this limit can be raised for an additional fee.