A weapon, in the most basic sense, is a system for concentrating energy in space and time. Armor, in the most basic sense, is a system for disbursing energy in space and time.
Let’s look at an example. A 5.56mm NATO round is a fairly small bullet. It’s also backed by a relatively large (for the bullet size) cartridge. That allows more propellent to be exploded to propel the bullet. In general a longer rifle barrel will allow the bullet to ‘capture’ more of the energy of the propellent.
When the bullet hits the target, all of the energy of the bullet is concentrated on the target in the very small area of the bullet. But also all that energy is concentrated in the microsecond(s) of impact.
Consider this. You don’t push an axe through a tree. You swing and hit the tree with the axe. All the energy that you have put into the axe by swing is concentrated in the edge and the instant of impact.
Indeed, with a little effort, you can calculate all this (with error for friction and some other details).
Armor on the other hand, is a system for taking that massive amount of energy that the weapon is projecting onto you and distributing it in both space and time so that the energy is reduced to some level that you can handle.
If you, as a human, are hit by that previously mentioned 5.56mm bullet, it will pretty much go straight through you. Same with the axe. Unarmored, you’re going to get severely hurt.
If you have an armored vest though, the bullet will impact that. The vest will attempt to do two things (three really, but two for our purposes). The first is that it will spread the energy from that tiny 5.56 bullet into an area about the size of your chest… say 0.25 square meters. For example, let’s say you get hit with 10,000 joule of energy in a 1mm^2 area. That’s a lot. But if the armor you are wearing distributes that energy to a 1000mm^2 area, then the energy per square millimeter is reduced to 10. (Assuming a perfect distribution.)
Second, the armor will slow the impact in time. The energy of the bullet will be used to deform the armor. That process happens much slower than the initial impact. Where the bullet might impact in a microsecond, the deformation of the armor might take several hundred milliseconds. Again, reducing the effective energy that is transmitted through the armor… to you. It’s going to hurt, there’s a lot of energy involved. But the bullet might not even get to you.
And that’s pretty much the history of modern weapons and armor. The weapons are able to concentrate more energy in time and space. The armor is more effective at dispersing that concentrated energy.