A Flak Jacket is an inexpensive form of ballistic cloth fashioned into a protective covering to guard the torso. A flak jacket offers limited protection against firearms. However, it will not defend you against personal energy weapons. 
Flak jacket tend to be originally developed in the TL:4-6 epoch to protect against shrapnel. They quickly grow increasingly sophisticated, almost immune to lesser projectiles. By TL:7-9, they can effectively block all but the larger rifle caliber rounds although with significant bruising or even broken bones from larger rounds. Bullets with greater penetration are created to defeat them. By TL:10-12, a good flak jacket can spread out and diminish kinetic energy. They are mostly obsolete by TL:13-15.