The Ethereum blockchain is working on a network of thousands of computers connected with each other exchanging information. Now, in order to guarantee that they all are processing the same transactions and working for the same goals as a combined group of efforts, we need something to reach agreements, which is called consensus, so that they all agree on the result. It's the best way to combine the efforts to benefit the whole.
In the following diagram, you can see how all the computers are running the same mining software while agreeing on a single protocol that provides them with a reward for their computing power:
