An alphanumeric sender ID is a text name of 3 to 11 letters, digits and spaces shown as the sender of an SMS instead of a number; recipients cannot reply to it.
A name as the sender is the most recognisable form of branding in SMS: the message appears under ACME rather than under a random number. Names are one-way, since there is no number to reply to, and they are accepted as typed in a large share of the world, notably most of Europe, Africa and parts of Asia.
Where operators require registration, an unregistered name is either replaced by a shared number or dropped; where names are not supported at all, a number is substituted. The country page is the reference.
At SMSMeteor: alphanumeric senders (3 to 11 characters, A-Z, 0-9, space) are delivered as typed in 114 destinations; the table by country lists every rule.