Problem is I know nothing about it! can somebody explain to me what it is in real life and its fallout impact?
It is a big cork in the Colorado River.
OK, the Hoover Dam is one of the few dams in the world that people can point to and say, the benefits of this dam far offset the environmental impact (which in this case was minimal). The damn farther upstream that created Lake Powell is still controversial in the benefit/impact equation. Most other dams wreck the watershed downstream, and upstream.
It provides a reservoir of Water that feeds most of the SW USA. Normally, the Colorado River is a trickle most of the year, running calm and smooth. During the Spring run off months, it can be like a flood. What the dam did was smooth out the flow of water so that Spring Runoff would be available the rest of the year. California, Nevada, Arizona would not be what they are today if it was not for the Hoover Dam. In fact, the populations would be one tenth to one quarter what they are now, and California would not be agricultural powerhouse that it is today.
The fact that the dam makes electrical power is almost secondary.
In the Fallout world, well it is still a source of water (always important) and it is a source of electricity (always important). As such, anyone who controls that, controls two of the most important things to a society. With Water and Energy all other things can be produced.