What is a dark sky reserve?
An IDA International Dark Sky Reserve is a public or private land possessing an exceptional or distinguished quality of starry nights and nocturnal environment that is specifically protected for its scientific, natural, educational, cultural, heritage and/or public enjoyment. Reserves consist of a core area meeting minimum criteria for sky quality and natural darkness, and a peripheral area that supports dark sky preservation in the core. Reserves are formed through a partnership of multiple land managers who have recognized the value of the natural nighttime environment through regulations and long term planning. ( http://darksky.org/idsp/reserves/)
It is basically a place which is free of Light Pollution and thus is an ideal place for stargazing and setting up astronomical observatories. With increase in population and increased human interference such places are diminishing.
To know about light pollution - http://www.readrevise.com/question.html?qrul=1638-&-what-is-light-pollution
Recently China launched its first Dark Sky Reserve at Ngari in Tibet near its India and Nepal borders.