SMPP (Short Message Peer-to-Peer) is the binary telecom protocol carriers and aggregators use to exchange SMS at high volume; most application senders use an HTTP API instead.
SMPP is the wholesale protocol of the SMS world. A client opens a persistent session (a bind) to a server, submits messages and receives delivery receipts over the same connection. It is efficient at thousands of messages per second and it is what gateways use towards carriers.
For an application, SMPP is rarely the right interface: it needs a long-lived connection, custom encoding of every field, and a library that handles binds, windows and reconnects. An HTTP API with JSON, idempotency keys and webhooks gives the same outcome with far less to maintain, up to volumes that batch endpoints handle comfortably.
At SMSMeteor: the public interface is a REST API (50 requests per second per key, 1,000 messages per API request); SMPP is used on the carrier side, not offered to accounts. The guide SMPP vs HTTP explains when you would actually need a bind.