Flak Armor
Flak Armor is the most common form of armor employed by the Imperium of Man and especially the Imperial Guard. Cheap and easy to make and manufacture by the average Imperial world, it is composed of multiple layers of ablative material, shock resistant material and ballistic textiles. Fairly similar to modern body armor in regards of rough construction, though the materials are more advanced. It is resistant to shrapnel and stab attacks done by human combatants armed with regular knife while offering pretty good protection against bullets from combustion guns and glancing hits from directed energy weapons comparable in power to a Lasgun.
For example, modern Earth ballistic vests either light weight vests that provide reasonable protection only against shrapnel and weaker rounds or poor penetrating (will stop a shotgun slug but broken ribs galore), or equip heavier armor that provide more coverage and withstand more powerful rounds, all the way up to armor piercing sniper rounds (assuming the wearer has some sort of steel or ceramic insert). With Flak armour gives a chance to not only survive a direct hit from a light machine gun, but to remain in a shape to continue fighting (assuming it hits the chest piece), all while not weighing down the soldier significantly. When dealing with a local rebellion or facing pirates, bandits, hive-gangsters or the rag tag forces of the lost and the damned, a suit of flak armor is an Emperorsend and often means the difference between survival and a gruesome death.
That said, when it comes to protecting its user from guns which shoot 20mm rocket bullets, hyper-sonic monomolecular ninja stars, bolts of compressed plasma as hot as a star's core and similar, they are of little more use than a common T-shirt.