As it applies to direct impingement style AR-15s: Stainless-AR-15-gas_tube-2

When a round is fired, the bullet travels down the barrel, propelled by gas. Once the bullet passes the gas port in the barrel (a tiny hole in one side of the barrel), the gas is forced through the gas port contained by the gas block, through the gas tube, and back into the upper receiver to cycle the next round in your magazine.

There are typically 4 different lengths of gas tube: pistol, carbine, mid-length, and rifle-length.

While most calibers prefer a longer gas system to cycle smoother, certain calibers like the 300 Blackout actually prefer a shorter gas system for proper cycling.