Short answers, pulled from the story.
The Great Game was a 19th-century rivalry between the British and Russian empires over influence in Central Asia, primarily in Afghanistan, Persia, and Tibet. The two colonial empires used military interventions and diplomatic negotiations to acquire and redefine territories, with Russia conquering Turkestan and Britain expanding and setting the borders of British India.
The term Great Game was coined in 1840 by British intelligence officer Captain Arthur Conolly. In a letter to Major Henry Rawlinson he wrote, "You've a great game, a noble game, before you," and Rudyard Kipling later popularized the phrase in his 1901 novel Kim.
The Great Game never erupted into a full-scale war directly between Russian and British colonial forces, despite distrust, diplomatic intrigue, and regional wars. The two nations did fight each other in the Crimean War from 1853 to 1856, which affected the Great Game.
Britain feared that Russia's southward expansion would threaten India, its most valuable colony. The British line of thinking held that if Russia gained control of Afghanistan, it could be used as a staging post for an invasion of India, so Britain made protecting all approaches to India a high priority.
By one major view the Great Game began on the 12th of January 1830, when Lord Ellenborough tasked Lord Bentinck with establishing a trade route to Bukhara, though historians disagree on the dating. It traditionally came to a close between 1895 and 1907, ending formally with the Anglo-Russian Convention of August 1907.
In 1838 the British marched into Afghanistan and deposed Dost Mohammad Khan, installing Shah Shuja Durrani, but the Afghans revolted by January 1842. During the British withdrawal a Kabul garrison of 4,500 troops and 12,000 camp followers was attacked by 30,000 Afghans, and only Dr William Brydon reached Jalalabad, an event that destroyed the force known as the Army of the Indus.