Simply, it means an original idea or creation and refers to the originators of that idea. Here are a few sentences to show you how this word is used:
The entire project was the brainchild of a small group of visionaries.
The report evaluated a "Pilot Community Policing Program" which was described as the brainchild of Chief Melekian and received high marks.
Ketcham's brainchild has been the subject of a hit network TV series, starring Jay North, which ran from 1959 to 1963, and still appears on stations around the country.
After years of work his brainchild was a tangible reality.