Armor level indicates at what setting of armor your dwarves will wear certain gear.
1 = leather
2 = chain
3 = plate
"ubstep" is how much of the upper body the gear covers.
0 = upper body
1 = upper body/upper arm
MAX = upper body/upper arm/lower arm
"lbstep" is lower body covering
0 = lower body
1 = lower body/upper legs
MAX = lower body/upper legs/lower legs
Well, that is for body armor.
[SHAPED] = craftable from shell
[HARD] = craftable from bone
[METAL] = craftable from metal
[SOFT] = craftable from cloth
[LEATHER] = craftable from leather
[SCALED] =
(craftable from scales but no scales in game?)
[BARRED] = 
[MATERIAL_SIZE] divided by 3 is how much material it takes to make something.
Mainblock = how much it blocks for the main part (e.g. upper body for chest, lower body for leggings)
Secondblock = how much it blocks for other things it covers (arms, legs, etc)
[METAL_ARMOR_LEVELS] means that the gear will be based on its material for dwarf preference rather than the [ARMORLEVEL] tag. Used for caps/helms currently.
[UPSTEP] is a tag on boots that indicates how far up the leg they go.
0 = feet only
1 = feet/lower legs
MAX = feet/lower/upper legs
[UPSTEP] on gloves is similar to boots.
0 = hands only
1 = hands/lower arms
2 = hands/lower/upper arms
I am not sure how the
[LAYER]
[COVERAGE]
[LAYER_SIZE]
[LAYER_PERMIT]
tags work although I know they pertain to layering on plate/chain/shirt/etc. I haven't played around with them enough to know.