See it this way. John met Chioma online and decided to go on a date with her, just the both of them. After the first date, the lust was doing him strong thing 'down-below' and he wanted to get into her honeypot sharp sharp so he told her 'he loves her and want to marry her', nobody heard him saying this to her. After getting what he wanted, he stopped picking her calls and denied saying he loved her. There was nobody to prove that he said he loves and want to marry her cos the conversation was between the both of them. This can be seen as a centralized system which can be breached.
Now imagine that when he told Chioma that he loves her and John friend, Richard, heard when he said it. If he denies it, Chioma can call Richard to prove he said it because he's a witness. Now, because of 'Bro Code', Richard got John back and denied that he ever said he love and want to marry her. This can be seen as a centralized system with backup servers which can also be corrupted.
Now what if Chioma had given John a task to prove that he loves her by standing in the middle of a crowd that has come to watch '50 shades of grey' at the cinema and saying it out loud to the hearing of everybody present. Since John has been thinking with his 'down-below' all day, he not only shouted it, he paid the Cinema to show it on their movie screen before the film starts. Remember this is a crowd that he doesn't know. Now, if after getting what he wanted and he try denying he said he loves and want to marry her, Chioma already has witnesses that have nothing to gain if they lie. So they just have to tell the truth of what they heard. This is what the Blockchain is about. A decentralized system.