STM is an application-layer communication protocol, built on top of a transport-layer protocol (e.g. TCP), to simplify the process of sending and receiving messages and data packets between different systems in an application. The list of messages and data items used in a communication link are defined as part of the application and exchanged between the two end points of the communication link when the connection is established.
Simple Messaging was originally called Simple TCP Messaging, therefore the STM moniker, however over time support for UDP as well as serial communication was added to the library.
The STM code contains a set of simple VIs which implement the API and currently VIs are provided for TCP, UDP and RS-232 as the transport layer. Due to its simple nature it can be easily implemented on other transport layers. For example if you want to send various messages or data items between a LabVIEW FPGA target and its host, you can implement STM using the DMA transfer as the transport layer.