A silo is a storage place for grain.
In 2151, the Klingon Klaang crash-landed in a cornfield in Broken Bow, Oklahoma, on Earth. He was pursued by two Suliban soldiers and ran into a silo owned by a farmer named Moore. The two Suliban followed Klaang inside the building but he escaped by jumping out of a door in the silo's upper section. He then destroyed the silo with a Klingon disruptor he had taken with him. (ENT: "Broken Bow")
The script of "Broken Bow" specifies that the gap between the bottom of the silo's ground-level door and the ground was six inches and that the upper door was "fifteen feet above ground." The shot of the silo exploding was scripted to involve visual effects. 
The silo and the scenes including it were filmed on location in Bakersfield, California. The silo was a full-size prop, actually constructed at the location. The reasoning why the structure was built especially for the episode was so it could be detonated on-site. ("Broken Bow" text commentary, ENT Season 1 DVD)
In contrast to the script, it was decided that no CGI would be incorporated into the shots of the silo exploding. Executive producer Brannon Braga noted, "If you watch the [episode] closely, you can see shock waves rippling through the corn. Nothing like a real, good, old-fashioned [...] fireball." ("Broken Bow" audio commentary, ENT Season 1 DVD)