JSMPP is a java implementation (SMPP API) of the SMPP protocol (currently supports SMPP v3.4).
SMPP stands for Short Message Peer to Peer. It is a standard protocol for exchanging SMS messages between SMS entities over TCP/IP or X.25 connections.
This API was created to reduce the verbosity of the existing SMPP API. It's very simple to use because it hides the complexity of the low-level protocol communication such as automatically enquire link request-response.
You can read more here.