Skill represents a character's proficiency, either with a particular weapon, or in a particular character trait or vocation. Skill can be enhanced by investing skill points, which are earned as you level up, or by using a seed of skill. Each character has five different skills available, however, in Dragon Quest IX, more skills become available in different vocations.
Once you have earned a certain number of skill points in a particular category, you gain either an ability unique to that weapon, character or vocation, or an attribute bonus. Bonuses gained from weapon skill are only effective while you wield that weapon; bonuses from character- or vocation-specific categories stay with you always, making these important to character progression.
100 is the highest number of skill points that can be put into any category. In Dragon Quest IX, gaining 100 skill points with a weapon or shields allows you to wield that weapon, or any shield, even in a vocation that normally cannot wield it. 100 skill points in Fisticuffs allows you to use Fisticuffs abilities and boosts in any vocation, while you do not wield any weapon. 100 skill points in a character- or vocation-specific category usually gives you an especially powerful ability.