In theory, the Blockchain is a man in the middle between buyer and seller and acts as a kind of bank. We all know banks and the money we leave with them when we make transactions and leave money with the bank and they become "too big to fail". To get around this problem the block chain was introduced, which works with a simple ledger. In this ledger all transactions like Bitcoin or other (crypto) currencies are recorded. The peculiarity of this ledger is, that every transaction that is recorded in the ledger causes a "mathematical problem". The "mathematical problem" can then be solved by one of the users in the block chain and records it in his ledger, whereby all other users of the block chain automatically copy the solution of the "mathematical problem". Each individual transaction (block entry) is thus recorded in the ledger and so that they are correctly linked together to form a "chain". This chain is mathematically balanced, so that fraud or incorrect postings are virtually impossible, because all other users of the block chain follow the invoice. In the future the length of the block chain can become problematic and that there are different block chains, which all have to be supplied with power. By the way, in my opinion the Blockchain has the potential to break for example large monopolies of search engines, because then the many individual users could be the search engine.
The MIT has a cool site to try and to see how it works!
The practise how the blockchain works is a little bit more complicated and in my opinion to exaggerated for a five minute explanation.

Sincerly
litshit69