Ketracel-white is a substance the Jem'Hadar need to survive, supplied by their Vorta overseers as a means of keeping the Dominion's soldiers loyal.
The Founders engineered the Jem'Hadar to depend on ketracel-white, an isogenic enzyme their bodies cannot produce. The Vorta control distribution, and a Jem'Hadar deprived of the white becomes violent and soon dies.
The dependency gave the Dominion an iron grip on its armies. In 2372 Julian Bashir met Goran'Agar, a Jem'Hadar who had somehow overcome the addiction and asked for help freeing his men. Bashir could not find a cure, suggesting that Goran'Agar's freedom was a fluke of his own biology.
The white also shaped battles. In 2374 a stranded Jem'Hadar unit whose supply was running out was sent into a hopeless attack by its Vorta commander, and Starfleet in turn targeted white facilities as strategic objectives.