Grit is an abstract calculation of your character’s ability to survive. It is part skill, part luck, part willpower, and part toughness. Hit points represent how much actual wounds your character can receive before dying.
Unless otherwise specified, if you ever take “damage”, the damage you take will be subtracted from your grit first, and your hit points once your grit reaches 0. For example, if you have 7 grit and 4 hit points, and you take 9 damage, you will have 0 grit and 2 hit points left.
Grit is easy to recover. Hit point damage is not.
Grit reaching 0 simply means your luck has run out, attacks direct at you are likely to cause wounds. However, If your hit points ever reach 0, you are dead.
When your character takes hit point damage, but does not die, you can elect to go unconscious. You will be unconscious for 1 hour per hit points you are missing (even if you are missing hit points beyond the would that caused you to elect to go unconscious).
But why would you ever do such a thing? An unconscious target is less of a threat, and going unconscious might allow you to live to fight another day.
Not that sometimes, you are not given a choice. A GM may inform you of a situation where you may go unconscious if you take hit point damage, or may require a save to stay conscious.
Some spells, features, or effects grant you “Temporary Grit”. Temporary grit is more of a damage buffer than actual grit. It is a pool of energy that protects you from taking the kind of damage that damages your grit.
Whenever you take grit damage while you have any temporary grit, the damage applies to your temporary grit first, and any left over damage will damage your actual grit normally. For example, if you have 9 grit, and 3 temporary grit, and you take 4 damage, you have 8 grit left.
Healing cannot restore temporary grit. If you have temporary grit from more than one source, you decide which source affects you, and the other source ends it’s effect.
Unless a feature states otherwise, your temporary grit lasts until lost, or until you finish a Night’s Rest.
Weapons, spells and other effects have the potential to deal damage to you. Unless specified otherwise, when you take damage, the damage you take is subtracted from your grit. Any damage beyond 0 grit is applied to your hit points.
When you hit with a weapon, you can roll the weapon's damage dice listed in the weapons entry. For example, an arming sword deals 1d6 points of damage. In addition, you can apply your Offence bonus to the damage roll of your weapon. If you are making a melee attack, or you are throwing a weapon, you can add your Strength Modifier to the damage of as well (or subtract your Strength penalty).
Ranged weapons do not gain an ability score modifier to damage, although they still apply your Offence Bonus. Weapons with the Finesse Weapon Feature still apply Strength to damage.