Consensus is a way of coming to agreements without a centralized authority. To reach consensus, all individuals in a group have equal say.
Most power structures are centralized, meaning that the power of decision making is “centered” at the top. This type of power structure isn’t a bad thing, de facto. It is more efficient and easier to achieve. Some examples of “centralized” power structures include monarchies, federal governments, central banks, and schools.
Consensus is found when a group comes to an agreed upon decision through collaboration and in the interest of the whole, not any individual, or those with power. Something called, “Consensus Protocol” is the overarching philosophy that powers Bitcoin transaction validations, new bitcoin improvement proposal (BIPs) upgrades, and forks. Some examples of organizations that use consensus (other than Bitcoin Core) are charities and cooperatives.