In one line, Communism means, “From each according to his ability, To each according to his needs.” Communism is basically a society or it is a way structuring people where there is no money (a moneyless society). It is a stateless society (with no borders have been drawn between countries) and a classless society (with no discrimination between the rich and the poor). Such a society where the means of production such as land, industries, factories, etc are operated and owned by the labourers i.e. the common public. 

Karl Marx is known as the father of Communism who was a German philosopher who published the Communist Menifesto in 1848. He grew up in an era when the industrial revolution had just begun. The labourers working in these factories worked in a very bad condition. The owners of these factories were often rich people who exploited their workers by making them work for maximum hours while giving them the least amount of payment and as usual keeping most of the profit with themselves. So according to Karl Marx, all society would settle into 2 classes: BOURGEOISIE (owners of the factory) and PROLETARIAN (workers). So he imagined a society where there are no differences between the rich and the poor. Basically he imagined a utopia (a perfect society that doesn't exits in reality) and named it Communism. 

In the book Communist Menifesto, he explained how can Communism can be achieved. He said that workers together can overthrow the form of government that exists forming a society where the means of production such as land, industries, factories, etc will not be owned by a single owner but rather the entire public. According to the Communist Menifesto, in the communist society, there won't be any class or discrimination between rich and poor or on the basis of race, religion. Free education and healthcare would be provided.There would be no inherited wealth. There will be no concept of private ownership.There would be even distribution of wealth. There won't be rich and poor and the society will have equality.

These things are quite idealistic but at the end of the day, everything is quite theoretical. It is nice to say but practically implementing them is near impossible. Although, we have seen some sort of communism (not the communism in its true sense) in few countries like China, North Korea, Cuba, Laos and Soviet Union but it usually leads to some sort of dictatorship and mass deaths of people who oppose the ideology.