MIDI (Musical Instrument Digital Interface) is a pretty old system. It's a simple serial protocol that dates back about 30 years. I remember a friend making an interface to hook up to the serial port of his computer, an Acorn Electron I believe. It still seems to be the default interface for sending note and other information between all sorts of gear. Modern kit may be sending it via USB or even within the PC to virtual instruments, but I think it's still pretty much the same.
About 15 years ago I bought a Casio CTK-530 keyboard to learn to play piano. The sounds are okay, but it has a five octave velocity sensitive keyboard. My kids have since also used it a lot. I think I have had it working with a PC via a cable that attached to the games port, but I recently realised this PC does not have that port. Luckily a friend had a spare USB MIDI adaptor that he sent me. It's an M-Audio Uno. I hooked it up, but nothing at all happened. A little research revealed I needed to install the midisport-firmware package. A quick reboot later I had some lights flashing on it.
I could see the Uno (as MidiSport) in the Jack ALSA tab. I wanted to get it playing sounds in Qsynth, but that is in the MIDI tab. I had to set the Jack MIDI driver to raw or seq to get the interface to appear as n output under MIDI. Then I could use my keyboard to trigger some of the nice sounds in the basic soundfont I have installed. I may have to look at some of the other sample libraries out there.
I also had to check if the Casio would receive MIDI. I loaded up a file in Rosegarden and selected the MidiSport as output. Out it came in spectacular cheesy 80s style. Not sure I need this capability for recording, but it's nice to have everything working.
This is actually the second piece of MIDI gear the studio has gained in the last week or so. I bought a Korg nanoKONTROL to give me some convenient control over the recording process. That really was plug and play. It was very simple to select it in Jack and then assign controls to any feature of Ardour. I need to experiment further with programming it using the Korg software that works on Linux via Wine. It has limitations, but it was cheap and fits on my crowded desk. I mentioned that I considered the Akai MPK Mini, but that cost a bit more and I didn't really need another keyboard.
There are still lots more Linux audio applications for me to experiment with. I did have a play with Rakarrack. It has a bewildering selection of effects with loads of parameters. I tried the presets, some of which didn't work. There's some interesting noises in there. I used their 'Satriani' preset on this track just to try it out.Satriani preset on rakarrak by steevc