What Is Consensus?

Bitcoin brought the idea that you should count the longest chain of blocks as the valid one. And so your security and consensus comes from people not being able to produce a longer chain than the main one. So that's also where a 51% attack comes from because now it's probably, probably, you will be able to produced a longer chain over time. As long as you can't produce a longerChain then we all agree that the longest chain is the one that we're counting is true. And so you have consensus there.