Hey I know the answer to this one!!
And don't be ashamed at 2 mins mate, took me forever to get there, and then about a month to 3:30
Packing is where you take a max breath, and then suck more air in, overpressurising your lungs. You'll see all these numbers around here like 'Lung Volume 8L, 10.5L full packing', that just means they can add an extra 2.5L of air over their maximum breathe in.
