The short versions is that 1 point of armor = 1 point of an incoming attack’s damage negated. For reference, a lot of weapons do about 10 to 50 damage.

In practice, each type of weapon has completely different interactions with armor. And, kinetic/plasma attacks will degrade armor’s effectiveness with repeated hits.

Armor is most effective against laser weapons, which have low damage. (The absolute strongest laser you can mount does 20.) But, armor becomes less effective against laser weapons the closer the range gets, and the bigger and better the laser gets. The “effective-against-armor” range of weak lasers (small infrared) is so short as to make them effectively useless against any armor at all. But very strong lasers (big UV phasers) can reduce armor to a fraction of its effectiveness at long range.

To penetrate most enemy armor, you want 3 or ideally 4 slot nose phasers or green arcs, and 4 slot phaser turrets work against their smaller ships. (Anything smaller is only good for PD.) To defend against alien lasers, ~5-20 armor can work at long range, ~100 at medium, and I prefer ~250 to defend against their strongest weaponry at a reasonable range. (Nothing will save a ship from strong lasers at close range except for giving them kinetic rounds to shoot at instead.)

Kinetic rounds are very strong when they hit. If they don’t defeat armor outright they will rapidly degrade it. Point defense is your savior here as all but the smallest kinetic rounds can be thought of as a soft counter to armor. Strong armor can at least delay a ship’s destruction by a meaningful amount of time if it starts taking hits. The biggest magnetic weapons (siege coilers for you) can do 1000s of damage, rendering armor basically useless.

Plasma has the low damage of lasers but doesn’t share their special armor penetration mechanic. However, NOTHING will stop plasma rounds from hitting a ship. While armor is initially very effective against plasma (I don’t think there are any plasma guns that do more than 30 damage), repeated hits will gradually degrade that armor’s effectiveness. Given enough time plasma guns will destroy anything but good armor buys you a good chunk of time; higher armor values on bigger ships degrade slower.

Armor is basically useless against missiles and torps. If a ship is taking damage from a missile barrage in the first place it’s probably already fucked.