Your armor has its own form of hit points called armor points (AP). Whenever you receive lethal damage, subtract from your AP before subtracting from your HP. Some tactics and sources of damage may specify that they "bypass armor"; in this case, your AP is unaffected and the damage reduces HP instead.
Armor & Recovery
Your armor is damaged if it’s received any damage; your armor is broken if its AP is 0. Armor points can only be recovered if your armor is repaired. It costs 1 gp to repair 1 AP. As long as you have gold, you can repair your armor in downtime. Spells and other magic may also repair armor points, and such magical recovery is instantaneous.
You can repair damaged armor by spending 1 gp. (For game purposes, we assume you have the equipment on hand to repair your armor.) It takes an hour per armor point to repair the armor. You can't do anything else while repairing your armor, such as rest to recover HP or wounds.
If your armor is reduced to 0 AP, then it is considered broken. Such armor provides no soak (or any other abilities, if it has any) until it is repaired.
Equipping armor takes time. For each point of AP the armor affords, it takes 1 round to equip it. You can take no other action while equipping armor and are considered prone.
Armor Points vs. Poison
In OSR+, you can administer a poison via a weapon attack against an armored opponent without having to do damage to their HP. Combat is fairly abstract in this system, so we fictionalize this by saying that your opponent uses finesse to get in between the chinks in your armor to administer the poison with their attack, but the weapon damage is still applied to your AP as normal for the sake of simplicity.
If no damage would be dealt from the weapon due to your opponent's soak, then your opponent is not affected by the poison.
Optional Poison Rules
If you're looking for more tactical depth, try these rules:
To administer poison via a weapon attack, you must make a called shot to bypass your opponent's armor. (If the opponent is not wearing armor, no called shot is necessary.) If successful, deal damage to their armor from the weapon as normal, and then administer the poison. Otherwise, just deal the damage to the armor.
If you intend to administer poison with a weapon attack against an armored opponent and want to target a particular part of their body, then you must make the attack at disadvantage (because you are making a called shot) and apply a -2 penalty, so as to account for how precise you've got to be in order to also administer the poison.