What is Consensus Mechanism in Blockchain System?

Human language is a good example of social consensus. Let us understand this by using some sentence. Alex asks Bob to pass salt on a dining table in English. There are implicit meanings in this statement.

1. Both Alex and Bob understand English.
2. Alex knows that this statement means that he desires to receive salt shaker.
3. Bob also understands what Alex desires.
4. Alex also believes that Bob understands the same meaning.

This means that there is a consensus on the requirements and understanding of the statement in given system. This is termed as common knowledge. Alex may not use the same language where English is not understood. The only reason for this is Alex knows that his statement would not be understood.

The similar concept applies in currency exchange. The only difference in currency exchange ecosystem is that the consensus is required in the complete ecosystem and not just the transacting parties. If I receive Fiat money or currency in exchange of goods and service, not only I but the complete ecosystem needs to recognize the same as money.

The additional requirement in the currency ecosystem is that it has to be scarce. The scarcity provides required motivation in the social ecosystem to render goods and services that are needed for the economy to sustain and operate.