Defence lower the damage you take from ennemies, you can see you have some defence when the attack of ennemies is in red. It scale mostly with items, and with some pots.

Scale : if you have 1 defence, all the ennemies attack will be lowered by 1, if you have 2 then it's lowered by 2. You can notice that a amplified attack (you know the "300% damage" thingy, it also lower it's damage by 300% of your defence so if you will be hit by a boss with 60 atk when you have 0 defence, he'll hit you for 180 hp, but if you have 1 in defence, he'll hit you for 177 hp)

Oh, wow, alot to learn, thanks for awnser!

Specifically, for the curious, the Defense stat is subtracted from every enemy's Attack stat. This happens before any other calculations, and indeed the number you see on-screen in the Maze already has the Defense subtracted from it. This means that adding to your Defense also reduces the amount of damage your summons/pets receive. And yes, the 300% damage dealt by certain creatures (most bosses) is calculated after the Defense stat is factored in, so it's 300% of the number you see on the creature in the Maze. 

Curse, by the way, acts exactly like a boost to Defense that only affects a specific enemy. So if you Curse a boss, it will also deal less damage to your pet, and deal less damage with its multiplied attacks. 

I used an item that +10 def but it didn’t reduce any damage whatsoever... anyone knows why?

Some things that should raise def sometimes have no effect.

There doesn't seem to be a cap, so I don't know why it works sometimes and not others.