Tabun gas is a secondary grenades that unleashes a poisonous smoke screen which blurs the vision and impairs movement. It is often used when the enemy is in a room or behind a wall, so that you can flank them.
Tabun gas is like a stun grenade, only better. When thrown, it releases deadly nerve gas that disorients whoever is in its radius. And boy do they work. They really mess up your vision and hearing, which makes fighting almost impossible. What makes them betters than stuns is that when you are stunned you can still see just fine. With Tabun Gas your camera is shaky, and its really hard to aim and even make out what weapon your holding! These are best for throwing into buildings, as whoever is inside will be dazed by the gas. To avoid Tabun Gas, use the Gas Mask perk.