The idea is that everyone should share the wealth equally. Everyone works and everyone earns the same. Theoretically this system involves "rule of the workers", in other words, very little government. There are several problems with this.
1) Whatever organization is in charge of distributing everything equally becomes the defacto government and has extreme powers whether it is supposed to be the government or not. This is why many communist nations end up with dictators.
2) You can't count on everyone contributing equally to society. Some people are disabled and some are lazy and will exploit the system.
3) Few people will want to make the effort to become medical doctors or engineers when they will get paid the same as laborers who did not have to spend lots of time and work really hard to gain their skills.
This system has still popped up around the globe because it can be made to sound quite good on paper even though it tends not to work in reality for the above reasons. It also sounds really good to poor people living under very oppressive regimes such as in Russia before the revolution. The most successful communist nations are the ones that mix some communist ideas with capitalistic ones like in China.