Armor is the way you protect yourself from other monsters or use skills.
Element Modifier and Defense Modifier[]
One of the purposes of armor is to give you protection. Element Modifiers will change depending on your armor. Sometimes, wearing an armor may make you more vulnerable to certain attacks. Also Defense Modifiers change depending on your armor. You may be easier to hit with Melee rather than Magic, and vice versa.
Classes and Skills[]
Most classes allow you to access skills by pressing the "Skills" button while wearing a certain armor given to you by purchasing it in its respective shop. After training, pressing "skills" will allow you to use a skill for SP , HP , or MP to use certain abilities for your class.