A seemingly simple question that perplexes many newcomers to audio and recording...MIDI was quite something when it first arrived in the early 1980s. It is still very much around today, although we don't tend to work quite as directly with it as before. MIDI (Musical Instrument Digital Interface) is now mostly hidden 'under the hood'. But still it is very useful to know about what it is and what it can do...
When you record an audio signal, then the acoustic or electronic waveform that the instrument produces is captured directly. The recording is a representation of the sound the instrument actually made, and will be different according to whether the instrument was, say, a violin or a trumpet. An audio signal is recorded on an audio track of a digital audio workstation software.
A MIDI signal is normally generated by a keyboard, and it contains information about which keys are being pressed. The MIDI signal can be recorded on a MIDI track of a digital audio workstation. Only the data about which keys were pressed, plus other associated data, is recorded. So the MIDI signal doesn't sound like a violin or a trumpet, it is merely a list of which keys were pressed and when.
To play back an audio signal, an audio output of the interface to your recording system must be connected to an amplifier or loudspeakers, or you can listen on headphones.
To play back a MIDI signal, a MIDI OUT connector on the interface to your recording system must be connected to the MIDI IN connector of a MIDI instrument or sound generator. The audio output of this instrument or sound generator must be connected to an amplifier or loudspeakers to make the sound audible.
[Alternatively, the MIDI signal can be routed within the workstation to a software instrument that will create an audio signal from it.]
MIDI seems more complicated then. Since it doesn't record the original sound, the MIDI signal must be connected to a sound generator to make it audible.
But this makes MIDI more versatile.
Your recording of a violin will always sound like a violin. You can EQ it, but it will still sound like a violin.
However, you might have had your keyboard set to a violin sound when you recorded your MIDI signal, but when you play it back you can set your keyboard to any sound you like. So what was once a violin can now very easily become a trumpet.
MIDI has further advantages...
You can edit the MIDI data more flexibly than audio. For instance you can correct the timing of notes, or how forcefully they were played. Or correct wrong notes even!
Also, you can record a whole composition in MIDI and then change the tempo. Audio is catching up in this respect, but there is always a sound quality loss.
MIDI, therefore, offers additional flexibility and conveniences. It has to be said that only synthesizers and sample-playback systems can respond to a MIDI signal. There is no such thing as a violin that will play from a MIDI signal. Yet.
But any newcomer to recording who can appreciate that an audio signal is a representation of the original sound, while MIDI contains only key press and associated data, will have come a long way in understanding.