Often I come across this word, but I do not fully understand what it means when applied to the blockchain. An agreement between miners?

Thanks everyone for the answers.
But after all, Bitcoin mixers are a third party, like exchanges, where are the guarantees that they will not rob me?

What is the essence of bitcoin mixers? Does it really make sense to use them only to those who conduct illegal activities?

What do I need to do to become a successful blockchain developer?