An American television series usually begins as a pilot, a single episode made to sell the show to a network. In the traditional cycle, each network hears about 500 pitches every summer, orders about 20 pilots, and chooses four to eight of them as series. By one estimate in Variety, only a little over a quarter of all pilots made for American television go on to become series.
The first pilot of Star Trek, “The Cage,” did not sell. NBC agreed to a second pilot, and that one became the series.