The Plus of an item is showed as +0 all the way to +9, It is a very important feature of an item. It will increase its stats. (only built-in stats with some exceptions, like Skill Dmg/Damage Bonus on certain weapons) (e.g. AC, AP, etc..)

As you upgrade the items to a higher plus they Can break but if they don't they are now stronger.

Some weapons gain more attack speed, while some Armor the required level increases as well.