I think it can be too easy to assume that everyone knows loads about how MIDI works and as a result just gets how virtual MIDI works too. I know that this isn’t the case, so if you’re struggling a bit with what MIDI is about you might be interested in Basic MIDI by Paul White.
It isn’t expensive on iBooks, and you can get a hard copy quite easily too. It isn’t massively up to date but the key MIDI concepts are all there and it’ll give you a clear idea about what people refer to when they’re talking about how MIDI works.
So if you’re struggling with MIDI, take a look.