What is area 51?

The answer for this question is bit complicated; But I did ...

Area 51 is a name used in official CIA documents (at least since 196) for a military base that is located in the southern portion of Nevada in the western United States, 83 miles (133 km) north-northwest of downtown Las Vegas. Situated at its center, on the southern shore of Groom Lake, is a large secretive military airfield. The base's primary purpose is to support development and testing of experimental aircraft and weapons systems.

The base lies within the United States Air Force's vast Nevada Test and Training Range. Although the facilities at the range are managed by the 99th Air Base Wing at Nellis Air Force Base, the Groom facility appears to be run as an adjunct of the Air Force Flight Test Center (AFFTC) at Edwards Air Force Base in the Mojave Desert, around 186 miles (300 km) southwest of Groom, and as such the base is known as Air Force Flight Test Center (Detachment 3)

The intense secrecy surrounding the base, the very existence of which the U.S. government barely acknowledges, has made it the frequent subject of conspiracy theories and a central component to unidentified flying object (UFO) folklore

Map showing Area 51, NAFR, and the NTS

Satellite view of Area 51 from 1968.

A-12 during radar testing at Groom Lake

Area 51 border and warning sign stating that "photography is prohibited" and that "use of deadly force is authorized"

A letter from the USAF replying to a query about Area 51

Have Blue/F-117 program

The Lockheed Have Blue prototype stealth fighter (a smaller proof-of-concept model of the F-117 Nighthawk) first flew at Groom in December 1977. Testing of a series of ultra-secret prototypes continued there until mid-1981, when testing transitioned to the initial production of F-117 stealth fighters. In addition to flight-testing, Groom performed radar profiling, F-117 weapons testing, and was the location for training of the first group of frontline USAF F-117 pilots. Subsequently, the still highly classified active-service F-117 operations moved to the nearbyTonopah Test Range Airport, and finally to Holloman Air Force Base.

The OXCART program

Even before U-2 development was complete, Lockheed began work on its successor as part of theCIA's OXCART project, involving the A-12, a Mach-3 high altitude reconnaissance aircraft — a later variant of which became the famed USAF SR-71 Blackbird. This aircraft flight characteristics and maintenance requirements forced a massive expansion of facilities and runways at Groom Lake in 1960 since the aircraft required an 8,000-foot (2,400 m) runway and additional infrastructure. The main runway was lengthened from 5,000 to 8,500 ft (1,500 to 2,600 m) in autumn 1960

And there are still more conspiracies which i'm not in the position to describe in this blog.