When you are a certain level, you can only wear a particular armor without it decreasing your attack or defense. Usually the weapons you can equip are five levels lower than the said required level. For example, a level 21 weapon can be used at level 16 without it decreasing your attack/defense; or a level 22 armor piece (boots, gloves, chestplate, leggings, helmet/hat) can be equipped at level 16 or 17. While you level up, it is easier to get better weapons and armor as you go (more defense and attack), also making it easier to level.