Midi Input is where your computer receives events from a Midi device (typically a piano keyboard or other instrument) for recording.

Midi Output is where your computer sends events to a Midi device (typically a sound card or other synthesizer) for playback.

Midi Thru is where events received on the input are immediately passed through to the output for playback.  This effectively makes a connection directly between the keyboard and the sound card, via the computer.

With Midi Thru, you can hear what you are playing, even if your keyboard does not have its own tone generator.  For keyboards which can generate their own sounds, it is still preferable to use Midi Thru so you hear the same tones that will be generated by the sound card during song playback.

Songtrix fully supports Midi Thru in two separate contexts.

Workspace Mode

In the workspace, you can have Midi Thru always active so that you can interactively explore sounds and musical ideas, or accompany the song playback.

In this situation, you have a choice of how the Midi Thru controller setup is managed, for aspects such as Channel Volume, Pan Position, Voice and Pitch Bend states.

Recording Mode

When recording events, the Midi Thru controller setup automatically uses the target track setup, so that as you play, you will hear exactly what the recorded events will sound like in playback.