Leech percentage is the amount of HP/MP you can regain of the damage you do to a mob per hit.
Imagine you equip one Life Absorb Ring+3 (+5% HP Steal). This means you steal 5% of the damage you do to an enemy per skill.
So if your skill does 500 damage to a mob, you would be able to regain 25 HP per skill from that mob.
Unfortunately this is not that simple and here is where the leech limit-cap comes into play.
The leech limit-cap is the maximum amount of HP/MP you can steal from an enemy per skill.
By default the base cap is 10% of your current level (Lv/10). So when you are lvl 140 you can steal a maximum of 14 HP/MP per skill.
So if we take the earlier example, you would be able to steal 25 HP, but because of the cap you only steal 14 HP. That's not too good.
To raise the cap you can equip items that add HP and/or MP per hit. These items have a line in their description that says "Max HP Steal per hit + X". This adds a certain amount of HP/MP to your leech limit-cap.
Now you add a Vampiric Earring+3 to your Life Absorb ring+3, This earring gives +25 HP/MP max steal, if you were to be Lv 140, your current leech cap would be: 14+25 = 39 (where 14 is your base Lv/10), thus now you would be leeching up to 39 HP/MP per hit, just keep adding until you're on your current max leeching capabilities.
Next to your Leech limit-cap, there is also a "Leech limit-cap"-cap. This basically means there is a max amount of HP/MP you can regain with leeching gear. This cap is at 5% of your max HP/MP. So if you have 2000 HP and 500 MP, you can leech a maximum of 100 HP and 25 MP. How much vampiric gear you add, this cap will not be affectable by anything (except raising your HP/MP with buffs or gear).
Let's say for an example that your base level is 120. With the equips you are wearing you have got Max HP Leech+15 and HP Leech 10%. With a certain spell you do 500 damage to a certain monster. Your total max HP Leech is (Lv/10)+(max HP Steal bonus) = (120/10)+15 = 12+15 = 27 HP If you use the spell which deals 500 damage, the max you could leech without your HP Leech cap would be 10% = 50 HP. Your HP Leech cap is 27 however, so you will only gain 27 HP. In this case it would be wise to get more equips which raise the cap of your HP leech. Try to get as close to that 50 HP Leech as you can to gain maximum benefit from your leech-items. However, you can only leech up to 5% of your HP/MP, so if your HP/MP is 2000 you can only leech up to 100 HP/MP per skill per mob.
Now lets say you are surrounded by a group of 5 mobs and you use a skill that damages each of them with 500 damage, that means you leech 5x27 HP = 140 HP per skill you use on that group. That way you heal yourself while damaging mobs.
* If you do not have any equips which give you HP Steal+X% you will leech 0% HP, which essentially means your vampiric equips are worthless.
* The HP/MP you regain from vampiric gear is not added to the damage you do already. You just take a part of the damage you do and regain your HP/MP with that.