China's Great Leap Forward was basically Mao's attempt to bring about socialist construction and bring communism to China. It was an effort to industrialize quickly, with hopes that China could surpass Western countries within 15 years (if not earlier).
The Great Leap Forward was one of the Three Red Banners, the other two being the General Line and the people's communes. The General Line sought to "go all out, aim high, and build socialism with greater, faster, better, and more economical". Developed by Mao and other leaders as an important concept during the Chengdu Conference (9-25 March 1958), it was officially formulated and established by the Chinese Communist Party (the CCP) in May 1958. It emphasized building a new communistic society as quickly as possible, stemming from the idea that the current generation will be the ones to build the communist society that they wanted to bring about. Prior to this, the state was spending large sums of money that they didn't have on industrialization (and getting very little back), and some leaders such as Zhou Enlai wanted to slow this down and opposed what they called "rash advance". However, Mao criticized the "opposition to rash advance" campaign, and Zhou Enlai would give self-criticism disavowing his opposition and declaring his loyalty to Mao's vision. As such, when the Three Red Banners were developed over the Chengdu Conference and later formulated during the Second Session of the Eighth National Conference, no one spoke up.
If the General Line was the idea of building a new communist society as possible, the Great Leap Forward was the action meant to build it. In order to build communist paradise as quickly as possible, the state set high production targets, most famously for crop yields and steel production. In order to do this, the large peasant population was ordered into large communes (with the average commune totaling over 5000 households) and directed to work on massive irrigation projects, deep plowing (which led to nutrient-rich topsoil being upturned in favor for nutrient-lacking deeper earth), close planting (or basically dumping a shit ton of seed in the belief that the denser the seeds in the soil, the higher the yield), backyard furnaces (often sourcing their metal from "scrap metal", which was basically almost every single metal object that they could get their hands on), ball bearing production (in order to use said ball bearings to make carts needed to move the vast amounts of earth being dug up through irrigation projects), factory work, and more. Communal kitchens opened up, with the idea that free food be available for all.
As to why the Great Leap Failed, I wrote a post about 2 months ago talking about it.
As to why millions of people died, this was because policies pursued during the Great Leap Forward led to mass starvation among the peasant population. I recently wrote up a huge ass post explaining this aspect of the Great Leap Forward, known as the Great Chinese Famine.