The Hindu Kush is a mountain range, a spur running southwest off the Pamir Knot, which is a large clump of high ground at the western end of the Himalayas. It is difficult country for travel; a disputed but plausible claim is that the name means "Hindu killer".
Most of the range is in northern Afghanistan; Band-e Amir National Park and the Afghan province of Nuristan are located within the Hindu Kush. The Wakhan Corridor leads from this range up into the Pamirs and to the Xinjiang autonomous region of China.
A hilarious and informative book about the area is Eric Newby's A Short Walk in the Hindu Kush; when Newby and a friend visited Nuristan in the 1950s, they were the second or third European expedition ever to reach it.