What is meant by the phrase ‘invisible hand’?

According to Adam Smith, the famous economist, the market economy is made up of a series of individual exchanges or transactions, which creates functioning and ordered system automatically. Every person has their own self-interest and the work to fulfil it. In fulfilling one's own self-interest, the interest of the society is fulfilled. Therefore, there is some sort of unseen force at work, which converts what is good for each individual into what is good for society. This unseen force was called invisible hand by Adam Smith.