Alright, so i havent played 40k in 10 years. Just picked up a copy of the 5th edition rule book to brush up and a lot is different. Im not sure if the AP weapon rules have changed at all, but for some reason i dont understand how they work.

I understand that it has some sort of modifier on the vehicles defensive roll, or what ever you would call it. So for instance, if a strength 10 AP one weapon is hitting the front armor of a land raider (14) what happends with the AP. I understand how the strength effects the roll to hit, but im unsure on the AP factor.

Also some weapons are AP4 or AP5, this is misleading for me, because it looks like it would be more piercing, however it is less. So how does this effect a vehicle? Anyway, any help would be nice, i pretty much understand every other rule/rule change with the 5th edition.