MIDI stands for Musical Instrument Digital Interface, a standard for sending and receiving performance information rather than sound itself. It records and transmits which pitch to play, when, how strongly, how controllers were moved, and more. Playing MIDI data alone produces no sound, since you need an instrument to make the sound.
Difference from audio
Audio is waveform data recorded from a microphone or instrument. MIDI is a set of instructions: notes, velocity, length, control changes. The same MIDI data sounds different depending on whether you send it to a piano instrument or a synth. File sizes are small, and tempo and key are easy to change afterwards.
Notes and velocity
Note information includes pitch, start position, and length. Velocity is a value representing how hard a key was struck, and some instruments use it for timbral change as well as volume. Controller information such as pitch bend, modulation, and pedal is also carried over MIDI.
Ways to enter it
You can record by playing a MIDI keyboard or pads, or draw notes into the piano roll with the mouse. Since you can edit timing, pitch, length, and velocity after the fact, you can keep writing even if you are not a strong keyboard player.
Relationship to instruments
Software instruments receive MIDI and output an audio signal. External synthesizers can be played over MIDI too, but the sound is produced by the hardware, so you need to record it into the DAW or bounce it in real time. Keep the roles of MIDI and audio separate in your mind.
Always check conversion results
Some tools estimate MIDI from recorded audio, but the analysis of pitch, chords, and rhythm is not always accurate. After converting, listen through the notes and correct them. Basic selection and movement in the DAW is covered in Tips for selecting and moving MIDI data (Japanese).
