I guess i’m going to start with MIDI (Musical Instrument Digital Interface). MIDI enables electronic instruments to communicate with a digital interface ie. your macbook pro. I’m guessing most people use a MIDI keyboard to play melodies onto their chosen software so ill explain that now.
What you’ll see on your arrangement window when you play in some notes will be a little rectangular box that indicates what note you played in there. The top half of the following picture shows the arrangement window and how midi looks in there.
MIDI not only reads what note you play in but at what velocity its played at (velocity meaning how hard you push the note). This is shown on the software in a change of colour. The bottom half of this picture shows the piano roll. In here you can move the notes around and shorten them as much as you want. This is why MIDI is industry standard.
I hope this helped? remember MIDI is just the communication between the instrument and the computer.