Soft body armor is a fairly mystifying concept: How can a soft piece of clothing stop bullets? The principle at work is actually quite simple. At its heart, a piece of bullet-proof material is just a very strong net.
To see how this works, think of a soccer goal. The back of the goal consists of a net formed by many long lengths of tether, interlaced with each other and fastened to the goal frame. When you kick the soccer ball into the goal, the ball has a certain amount of energy, in the form of forward inertia. When the ball hits the net, it pushes back on the tether lines at that particular point. Each tether extends from one side of the frame to the other, dispersing the energy from the point of impact over a wide area.
The energy is further dispersed because the tethers are interlaced. When the ball pushes on a horizontal length of tether, that tether pulls on every interlaced vertical tether. These tethers in turn pull on all the connected horizontal tethers. In this way, the whole net works to absorb the ball's inertial energy, no matter where the ball hits.
If you were to put a piece of bulletproof material under a powerful microscope, you would see a similar structure. Long strands of fiber are interlaced to form a dense net. A bullet is traveling much faster than a soccer ball, of course, so the net needs to be made from stronger material. The most famous material used in body armor is DuPont's KEVLAR fiber. KEVLAR is lightweight, like a traditional clothing fiber, but it is five times stronger than a piece of steel of the same weight. When interwoven into a dense net, this material can absorb a great amount of energy.
In addition to stopping the bullet from reaching your body, a piece of body armor also has to protect against blunt trauma caused by the force of the bullet. In the next section, we'll see how soft body armor deals with this energy so that the wearer doesn't suffer severe injuries.