The term 'invisible hand' was first used by Adam Smith in the 18th century to describe the efficiency in the autoregulation of free markets, where free means not externaly intervened by governments. Intervention distorts markets and harms the efficiency of their functioning.
The main principle behind is that, when individuals pursue their "private" efforts they are generating larger wealth for society as a whole than if their first intention was to help society. The markets will be in charge of channelizing individuals interests into socialy desirable outcomes.