Just discovered this wonderful world: 'modulAR' appx 2 wks ago, and it's like going back to when I used to build PCs for fun....in the 80s/90s and when I discovered Linux, that was a blast man as is this.

So, here goes with the clottish Q: Please define HP - what is it - a measure of width(depth/height) - all answers greatly appreciated.....

HP stands for horizontal pitch.
It is the width of a module.

HP on Wikipedia

"Hit Points". It determines the maximum health of your module. You can battle monsters to increase it. The higher it is, the crazier the sounds you can make.

hp is a measure of how much space a module takes on a rack. So if a rack holds 84 hp, you can only have enough modules which total hp equals 84

just kidding hp is happy peperoni