Interesting plugin.

To make it simple, you can increase a ln actor's mastery towards an equippable weapon. But what does it do?

Higher mastery -> higher damage?

I suppose it's customizable, but what's the default?

Mastery can modify base stats (HP, mp, atk, def, mat, AGI, luk), ex stats such as hit, Eva, crit, crit Eva, mag reflect, mag evade, regents, sp stats such as exp, mp cost rate, tp charge rate, etc.

Damage can be lower or higher but stat effects come into effect when equipment type is equipped 

Oh right, there's also skills based on equip level too