To understand 'blockchain', first, you need to grasp what is a block.
A block is just like a ledger. Each new transaction that takes place on Bitcoin is recorded in a new block. Every block contains the hash of the previous block. Hash means the random sequence of letters and numbers. This way, any transaction made today through Bitcoin can be traced back to the genesis block(the first block of a blockchain). In other words, each new block contains the summary of previous blocks and due to this the size of blockchain continually grows in size which until now has reached 159.08 GB. Additionally, every new block is generated chronologically because it contains the hash of a previous block. This is how it becomes a chain of blocks and thus called as 'The Blockchain'.
The blockchain, as a whole, is known as an open ledger. Banks are probably afraid of this completely transparent and fastly growing technology. Blockchain system also offers pseudonymity because the name and address of the parties involved in the transaction are not disclosed rather there is an Alphanumeric wallet address only. Maybe this is the reason why the government financial authorities of China and South Korea are imposing bans on ICOs.
Image Source:- https://upload.wikimedia.org/wikipedia/commons/b/bb/Broad_chain_closeup.jpg
