Shroud is a term used to denote the ability of the Jem'Hadar to "cloak" themselves to avoid capture, injury, and to confound their opponents. A shrouded Jem'Hadar would seem to vanish into thin air, although this was not the case. When they used the shroud, the immediate area in which they stood would turn dark, then become almost perfectly transparent. Only the most observant individuals could see the slight shifts in light as the Jem'Hadar moved about in their shrouded condition. (DS9: "To the Death")
Like all Jem'Hadar functions, the ability to shroud is regulated by the presence of ketracel-white. Jem'Hadar who endure prolonged periods without the drug eventually lose their ability to shroud. Such a degeneration in their abilities signals severe withdrawal from the addiction to ketracel-white, and indicates that death is not far off. (DS9: "Rocks and Shoals")