Components can use the queue.push function to push messages into the queue for the component.
These messages are received by all the components that have connections from that component (see Linking components). This is the Lua code to call it:
local MessageId = queue.push{data=Data}
This function returns a unique string identifier (the message ID) which can be used to link messages so you can see actions performed on a message in the Logs.
See Custom Logging for more information on this.