In Command & Conquer, Shroud is a battlefield condition that prevents you from knowing anything about the terrain of an area (as well as the units in it) until you scout it out (the area was covered in black). Because of this, it is recommended that a commander does not send very expensive units into shroud without scouting out the area first or unless they were certain that they would be safe (or unless they would at least be able to damage an enemy to compensate).
In earlier games, shroud was always present, and fog of war was not on by default (there was no option to turn it on in some games). In Command & Conquer: Generals and games after that, this was reversed in skirmish - with fog of war always being on and shroud always being off. However, shroud conditions still affected the maps in campaigns.
Scout units, such as Pitbulls, are capable of quickly revealing shroud. This is also the case with airborne units, from Generals onwards.