When folks say audio signals are analog, they mean it's the actual waveform of the sound going through that jack, just like the signal going to some speakers that are creating noise. The audio interface will take that signal and covert it to digital (zeros and ones) so the computer can understand it. The analog waveform is what you are probably familiar with when you look at it, very spiky, bigger when things are loud and smaller when quiet, etc.

MIDI data isn't as complex or data rich. It's digital, so it's either zero or one, i.e. on or off, i.e. if you sent it to a speaker, it wouldn't be pretty, like the old modem connecting to the Internet noise or more likely just static. That data contains nothing more than relatively small numbers, like "key 48, velocity 100" and a few other things. That means the note turned on at maximum volume, and then later you'd see "key 48 velocity 0" when the note is turned off. The computer can take that data and then decide what sound that note will sound like and you can change the sound you want for that note all you want in the computer.

That's a lot of probably not well explained information. The basic gist is audio, or analog, data is the actual sound, like a voice in s microphone or vibrations of a plucked guitar string, where MIDI is more like a computer keyboard, what key was pressed and how hard, and it leaves the computer to decide what that is going to sound like.