When Bitcoin was first proposed and created its biggest achievement was creating first functional electronic money. That all was enable because of different achievement in the field, thanks to the bitcoin and creation of Blockchain. Achieving trust in a decentralised system. Such trust is called consensus. You can think of consensus as a way to reach agreement between a set of people. In terms of a blockchain, except of people you have nodes in the network that need to reach an agreement on a final state of data, the so called truth.
But to achieve such consensus in a blockchain network you need an algorithm which will help with that. Such consensus algorithms are also called a consensus mechanism. Different names but they refer to the same thing.