The Long March was a military retreat undertaken by the Red Army of the CCP to evade the pursuit of the KMT army. There wasn't one Long March, but a series of marches, as various Communist armies in the south escaped to the north and west. The most well known is the march from Jiangxi province which began in October 1934. The First Front Army of the Chinese Soviet Republic, led by an inexperienced military commission, was on the drink of annihilation by Heneralissimo Chiang Kai-shek's troops in their stronghold in Jiangxi province. The CCP, under the eventual command of Mao Zedong and Zhou Enlai, escaped in a circling retreat to the west and north, which reportedly traversed some 12,500 K over 370 days. The route passed through some of the most difficult terrain of western China by traveling west, then north, to Shaanxi.
Mao increased his communist supporters drastically through out the Long March. The long march seems to be the turning point of CCP coming to power.