Hi Clinton,
Maybe this introduction to MIDI series will help:
https://www.youtube.com/watch?v=TVJ0m3f0pjQ There are three different parts to this series.
Basically MIDI CCs, continuous controllers, control MIDI devices. Some CCs are designated, like CC7, CC11 and CC2 are for volume, CC10 is for pan etc. Many hard and soft synths have other designated CC numbers that can control filters, effects etc. Each hard and soft synth will have a MIDI Implementation Chart that will show you what CC numbers that synth will respond with and what they control. All CCs can be assigned to anything you want them but it is
very wise to keep those that are universally assigned like CC7, CC11, CC2, CC10 etc.
All of the better hard and soft synths have velocity layers. Most acoustic instruments will have different sounds if played soft to loud. If you have ever played one you know what I mean. Velocity layers emulate that subtle tonal change.
I wouldn't worry about velocity layers until you get a grip on MIDI CCs.
I hope this helps - MarioD