Southern Nevada is a region of Nevada. It comprises Clark County, Esmeralda County, Lincoln County, Mineral County, and Nye County.
If you are going into the desert areas, be aware of all the usual precautions, but also two special precautions -- first, areas close to Rachel belong to the US government as the Nellis Air Force Base complex (and also contain the top-secret base popularly known as Area 51), and entry by unauthorized persons is strictly prohibited. The areas are well-marked with signposts marking the boundaries, and even though you most likely will not see guards, they will be watching you.
Associated with this is that in the '50s, Southern Nevada was used as a nuclear test range (before the risks of open-air tests became widely known, and before the Comprehensive Test-Ban Treaty was ratified, banning open-air tests), as the area was relatively uninhabited. There are regions which are still contaminated by radioactive fallout (as infamously demonstrated in the filming of the John Wayne film The Conqueror, which was filmed downrange of a nuclear test). Check with local authorities to make sure that where you want to go is safe.