How MIDI Works
MIDI ("musical instrument digital interface") was invented in 1982 as a "language" for digital synthesizers to communicate with each other. As MIDI gained in popularity the MIDI standard was adopted by a range of devices: from music keyboards to drum machines to recording gear to stage lighting. As digital recording became commonplace and the desktop computer became central to the recording experience, engineers longed to a more tactile experience the desktop computer lacked.
To solve this problem, MIDI controllers were created that mimicked the interfaces of old analog mixing consoles. But instead of audio circuits controlled by analog faders and knobs, the new controllers used digital faders and encoders to send commands (volume up/down, pan left/right, EQ settings, etc.) to the desktop computer... and to send all those commands, it used the communication protocol called MIDI.
When you connect a MIDI controller to your computer and turn a knob, press a button or slide a fader, the MIDI controller simply sends a command to your computer letting it know which control was altered and by how much it was altered. Conversely, a MIDI-aware program on your computer can send MIDI comannds back to the MIDI controller to tell the Panel, for example, to update it's own fader position.
This Is Where PFixer Comes In
To describe it simply: PFixer acts as a communication bridge between a physical MIDI controller connected to your computer and Adobe Lightroom.
As it receives MIDI messages from the MIDI controller, it translates those messages into actions inside of Lightroom, and as changes are made inside of Lightroom, it sends notifications back to the MIDI controller when needed.