Phosphine gas (PH3) is a deadly gas created by a mixture of red phosphorous and moisture in the presence of heat.
While Walter White's on a ride-along with his DEA brother-in-law Hank Schrader, Hank mentions that a meth lab can be dangerous on a good day because if you mix the wrong chemicals, you get mustard gas. Walt corrects it to phosphine gas which Hank comments can kill you if you get one whiff of it.
Later, when forced to cook meth by Krazy-8 and Emilio Koyama, Walt mixes red phosphorous into a pan of water he has over a burner after taking and holding a deep breath. While Walt escapes, Krazy-8 and Emilio fall victim to the phosphine gas created by the combination. Walt holds the door closed so to prevent escape, and both men eventually succumb to the poisonous gas. Due to the danger of the lingering gas, Walt rushes from the scene with gas masks on both himself and the unconscious Jesse Pinkman. He subsequently explains his use of the phosphine gas to Jesse when he wakes up. ("")
Walt and Jesse subsequently discover that while the gas killed Emilio, Krazy-8 somehow survived and temporarily managed to escape before being recovered by Walt. The gas appeared to have burned him and damaged his brain and respiratory system, judging from his disoriented state, wheezing breaths, and raspy voice. However, he ultimately remains alive and Walt admits that he isn't sure whether or not the gas will kill him through a delayed effect. The disorientation eventually subsides and Krazy-8's voice eventually returns to normal, suggesting the damage was not permament. (""). Walt is subsequently forced to strangle Krazy-8 with a bike lock. ("")
Although in the show Walt makes phosphine by dropping red phosphorous in hot water, which also produces a visible explosion, in reality this would not work. Red phosphorus does not hydrolyze to any appreciable degree in hot water, nor does it ignite. Metal phosphides, on the other hand, would rapidly hydrolyze in the presence of water, and some, like calcium phosphide, also generate diphosphane in contact with water, which is pyrophoric and whose rapid ignition in air would appear as an explosion (this is how technical-grade calcium carbide ignites in contact with water). Although Walter clearly uses red phosphorus, it's safe to assume that the writers decided to simplify the plot of him getting rid of the mobsters, by having him use something he had on the RV cooking table, rather than pull some metal phosphides out of nowhere.
- Phosphine on Wikipedia