Shroud is the term used to describe the Jem'Hadars' ability to cloak themselves. This ability gives the Jem'Hadar a great tactical advantage on the battlefield.
A Jem'Hadar's ability to shroud is affected by the supply of ketracel-white within their system. Jem'Hadar low on white often lose the ability to shroud. (DS9: "Rocks and Shoals")